Examine individual changes

'{{short description|Hypothetical form of matter}} {{distinguish|Antimatter|Dark energy|Dark fluid|Dark flow}} {{other uses}} {{Use dmy dates|date=September 2019}} {{Physical cosmology|comp/struct}} <!-- Before changing percentages, please note that 85% here refers to that of matter /excluding/ dark energy: (Dark matter 26.8%, Dark energy 68.3%, Ordinary matter 4.9%, Total 95.1% is Dark matter & Dark energy)--> '''Dark matter''' is a hypothetical form of [[matter]] thought to account for approximately 85% of the matter in the [[universe]].<ref name="NASA Dark Matter">{{cite web |url=https://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy| title=NASA Science Universe – Dark Energy, Dark Matter |website=NASA Science |access-date=23 May 2021}}</ref> Dark matter is called "dark" because it does not appear to interact with the [[electromagnetic field]], which means it does not absorb, reflect, or emit [[electromagnetic radiation]] and is, therefore, difficult to detect. Various [[astrophysical]] observations{{snd}}including [[gravitation]]al effects which cannot be explained by currently accepted theories of [[gravity]] unless more matter is present than can be seen{{snd}}imply dark matter's presence. For this reason, most experts think that dark matter is abundant in the universe and has had a strong influence on its structure and evolution.<ref name="CERN Dark Matter">{{cite web |url=http://home.cern/about/physics/dark-matter |title=Dark Matter |website=CERN Physics |date=20 January 2012}}</ref> The primary evidence for dark matter comes from your mom and calculations showing that many [[Galaxy|galaxies]] would behave quite differently if they did not contain a large amount of unseen matter. Some galaxies would not have formed at all and others would not move as they currently do.<ref name="Siegfried">{{Cite news |last=Siegfried |first=T. |date=5 July 1999 |title=Hidden space dimensions may permit parallel universes, explain cosmic mysteries |url=http://www.physics.ucdavis.edu/~kaloper/siegfr.txt |work=The Dallas Morning News}}</ref> Other lines of evidence include observations in [[gravitational lensing]]<ref name="Trimble 1987">{{cite journal |last=Trimble |first=V. |date=1987 |title=Existence and nature of dark matter in the universe |journal=[[Annual Review of Astronomy and Astrophysics]] |volume=25 |pages=425–472 |bibcode=1987ARA&A..25..425T |doi=10.1146/annurev.aa.25.090187.002233 |s2cid=123199266 |url=https://cloudfront.escholarship.org/dist/prd/content/qt2hz008rs/qt2hz008rs.pdf |archive-url=https://web.archive.org/web/20180718231719/https://cloudfront.escholarship.org/dist/prd/content/qt2hz008rs/qt2hz008rs.pdf |archive-date=2018-07-18 |url-status=live}}</ref> and the [[cosmic microwave background]], along with astronomical observations of the [[observable universe]]'s current structure, the [[Galaxy formation and evolution|formation and evolution of galaxies]], mass location during [[galactic collision]]s,<ref>{{Cite web |url=https://arstechnica.com/science/2017/02/a-history-of-dark-matter |title=A history of dark matter |year=2017}}</ref> and the motion of galaxies within [[galaxy cluster]]s. In the standard [[Lambda-CDM model]] of cosmology, the total [[Mass–energy equivalence|mass-energy]] content of the universe contains 5% ordinary matter, 26.8% dark matter, and 68.2% of a form of energy known as [[dark energy]].<ref name="NASA Planck Mission">{{cite web |url=http://www.nasa.gov/mission_pages/planck/news/planck20130321.html |title=Planck Mission Brings Universe into Sharp Focus |website=NASA Mission Pages |date=21 March 2013}}</ref><ref name="NASA Science Dark Matter">{{cite web |url=https://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy/ |title=Dark Energy, Dark Matter |website=NASA Science: Astrophysics |date=5 June 2015}}</ref><ref name="planck_overview">{{cite journal |first1=P.A.R. |last1=Ade |first2=N. |last2=Aghanim |first3=C. |last3=Armitage-Caplan |collaboration=Planck Collaboration |title=Planck 2013 results. I. Overview of products and scientific results – Table&nbsp;9 |journal=[[Astronomy and Astrophysics]] |volume=1303 |page=5062 |url=http://www.cosmos.esa.int/web/planck/publications |date=22 March 2013 |arxiv=1303.5062 |bibcode=2014A&A...571A...1P |doi=10.1051/0004-6361/201321529 |s2cid=218716838 |display-authors=etal}}</ref><ref name="wmap7parameters(a)">{{cite web |title=First Planck results: the Universe is still weird and interesting |url=https://arstechnica.com/science/2013/03/first-planck-results-the-universe-is-still-weird-and-interesting/ |author=Francis, Matthew |date=22 March 2013 |website=Ars Technica}}</ref> Thus, dark matter constitutes 85%<ref group="lower-alpha">Since dark energy does not count as matter, this is {{nowrap|{{sfrac|26.8|4.9 + 26.8}} {{=}} 0.845}}</ref> of the total mass, while dark energy and dark matter constitute 95% of the total mass-energy content.<ref name="planckcam">{{cite web |url=http://www.cam.ac.uk/research/news/planck-captures-portrait-of-the-young-universe-revealing-earliest-light |title=Planck captures portrait of the young Universe, revealing earliest light |date=21 March 2013 |publisher=University of Cambridge |access-date=21 March 2013}}</ref><ref name="DarkMatter">{{cite book |first=Sean |last=Carroll |year=2007 |publisher=The Teaching Company |title=Dark Matter, Dark Energy: The dark side of the universe |at=Guidebook Part&nbsp;2 p.&nbsp;46 <!-- access-date=7 October 2013 --> |quote=...&nbsp;dark matter: An invisible, essentially collisionless component of matter that makes up about 25&nbsp;percent of the energy density of the universe ... it's a different kind of particle... something not yet observed in the laboratory&nbsp;...}}</ref><ref>{{cite magazine |title=Dark matter |magazine=National Geographic Magazine |department=Hidden cosmos |url=http://ngm.nationalgeographic.com/2015/01/hidden-cosmos/ferris-text |access-date=10 June 2015 |first=Timothy |last=Ferris |date=January 2015}}</ref><ref name="wmap7parameters">{{cite journal |last1=Jarosik |first1=N. |display-authors=etal |date=2011 |title=Seven-year Wilson microwave anisotropy probe (WMAP) observations: Sky maps, systematic errors, and basic results |journal=[[Astrophysical Journal Supplement]] |volume=192 |issue=2 |page=14 |arxiv=1001.4744 |bibcode=2011ApJS..192...14J |doi=10.1088/0067-0049/192/2/14|s2cid=46171526 }}</ref> Because no one has directly observed dark matter yet{{snd}}assuming it exists{{snd}}it must barely interact with ordinary [[Baryonic matter#Baryonic matter|baryonic matter]] and radiation except through gravity. Dark matter is thought to be non-baryonic; it may be composed of some as-yet-undiscovered [[subatomic particle]]s.<ref group="lower-alpha">A small portion of dark matter could be baryonic and/or [[neutrino]]s. See [[Baryonic dark matter]].</ref> The primary candidate for dark matter is some new kind of [[elementary particle]] that has [[Physics beyond the Standard Model|not yet been discovered]], particularly [[weakly interacting massive particles]] (WIMPs),<ref name="Copi 1995"/> Many experiments to detect and study dark matter particles directly are being actively undertaken, but none have yet succeeded.<ref name="bertone hooper silk" /> Dark matter is classified as "cold," "warm," or "hot" according to its [[velocity]] (more precisely, its [[free streaming]] length). Current models favor a [[cold dark matter]] scenario, in which [[Structure formation|structures emerge]] by the gradual accumulation of particles. Although the scientific community generally accepts dark matter's existence,<ref>{{cite journal|url=https://www.scientificamerican.com/article/is-dark-matter-real/|title=Is dark matter real?|date=August 2018|author1=Hossenfelder, Sabine |author2=McGaugh, Stacy S|journal=Scientific American|volume=319|issue=2|pages=36–43|doi=10.1038/scientificamerican0818-36|pmid=30020902|bibcode=2018SciAm.319b..36H|s2cid=51697421}} "Right now a few dozens of scientists are studying modified gravity, whereas several thousand are looking for particle dark matter."</ref> some astrophysicists, intrigued by specific observations that are not well-explained by ordinary dark matter, argue for various modifications of the standard laws of [[general relativity]]. These include [[modified Newtonian dynamics]], [[tensor–vector–scalar gravity]], or [[entropic gravity]]. These models attempt to account for all observations without invoking supplemental non-baryonic matter. {{Beyond the Standard Model|expanded=Evidence}} {{toclimit|3}} == History == === Early history === The hypothesis of dark matter has an elaborate history.<ref>{{cite journal |last1=de Swart |first1=J.G. |last2=Bertone |first2=G. |last3=van Dongen |first3=J. |title=How dark matter came to matter |journal=Nature Astronomy |date=2017 |volume=1 |issue=59 |page=0059 |doi=10.1038/s41550-017-0059 |arxiv=1703.00013 |bibcode=2017NatAs...1E..59D|s2cid=119092226 }}</ref> In a talk given in 1884,<ref>{{Cite web|url=https://ned.ipac.caltech.edu/level5/Sept16/Bertone/Bertone2.html|title=A History of Dark Matter – Gianfranco Bertone & Dan Hooper|website=ned.ipac.caltech.edu}}</ref> [[William Thomson, 1st Baron Kelvin|Lord Kelvin]] estimated the number of dark bodies in the [[Milky Way]] from the observed velocity dispersion of the stars orbiting around the center of the galaxy. By using these measurements, he estimated the mass of the galaxy, which he determined is different from the mass of visible stars. Lord Kelvin thus concluded "Many of our supposed thousand million stars, perhaps a great majority of them, may be dark bodies".<ref>{{cite book |last1=Kelvin |first1=Lord |title=Baltimore Lectures on Molecular Dynamics and the Wave Theory of Light |date=1904 |publisher=C.J. Clay and Sons |location=London |page=274 |url=https://babel.hathitrust.org/cgi/pt?id=ien.35556038198842&view=1up&seq=304}}</ref><ref name=":1">{{cite news |url=https://arstechnica.com/science/2017/02/a-history-of-dark-matter/ |title=A history of dark matter |website=Ars Technica |access-date=8 February 2017 |language=en-us}}</ref> In 1906, [[Henri Poincaré]] in "The Milky Way and Theory of Gases" used the French term {{lang|fr|matière obscure}} ("dark matter") in discussing Kelvin's work.<ref>{{cite journal |last1=Poincaré |first1=H. |title=La Voie lactée et la théorie des gaz |journal=Bulletin de la Société astronomique de France |date=1906 |volume=20 |pages=153–165 |url=https://babel.hathitrust.org/cgi/pt?id=uiug.30112110949630&view=1up&seq=171 |trans-title=The Milky Way and the theory of gases |language=fr}}</ref><ref name=":1" /> The first to suggest the existence of dark matter using stellar velocities was Dutch astronomer [[Jacobus Kapteyn]] in 1922.<ref>{{cite journal |title=First attempt at a theory of the arrangement and motion of the sidereal system |first=Jacobus Cornelius |last=Kapteyn |author-link=Jacobus Kapteyn |journal=Astrophysical Journal |volume=55 |pages=302–327 |year=1922 |quote=It is incidentally suggested when the theory is perfected it may be possible to determine ''the amount of dark matter'' from its gravitational effect. |bibcode=1922ApJ....55..302K |doi=10.1086/142670}} (emphasis in original)</ref><ref name="Patras2014">{{cite conference |title=Status of the Axion Dark-Matter Experiment (ADMX) |first=Leslie J |last=Rosenberg |url=http://indico.cern.ch/event/300768/session/0/contribution/30/attachments/566901/780884/Rosenberg-Patras_30jun14.pdf |archive-url=https://web.archive.org/web/20160205163816/http://indico.cern.ch/event/300768/session/0/contribution/30/attachments/566901/780884/Rosenberg-Patras_30jun14.pdf |archive-date=2016-02-05 |url-status=live |date=30 June 2014 |page=2 |conference=10th PATRAS Workshop on Axions, WIMPs and WISPs |conference-url=http://axion-wimp2014.desy.de}}</ref> A publication from 1930 points to Swedish [[Knut Lundmark]] being the first to realise that the universe must contain much more mass than we can observe.<ref>{{Cite journal|last=Lundmark|first=K.|date=1930-01-01|title=Über die Bestimmung der Entfernungen, Dimensionen, Massen und Dichtigkeit fur die nächstgelegenen anagalacktischen Sternsysteme.|url=https://ui.adsabs.harvard.edu/abs/1930MeLuF.125....1L|journal=Meddelanden Fran Lunds Astronomiska Observatorium Serie I|volume=125|pages=1–13|bibcode=1930MeLuF.125....1L}}</ref> Dutchman and radio astronomy pioneer [[Jan Oort]] also hypothesized the existence of dark matter in 1932.<ref name="Patras2014" /><ref>{{cite journal |author=Oort, J.H. |year=1932 |title=The force exerted by the stellar system in the direction perpendicular to the galactic plane and some related problems |journal=Bulletin of the Astronomical Institutes of the Netherlands |volume=6 |pages=249–287|bibcode=1932BAN.....6..249O }}</ref><ref>{{cite web |title=The hidden lives of galaxies: Hidden mass |website=Imagine the Universe! |url=http://imagine.gsfc.nasa.gov/teachers/galaxies/imagine/hidden_mass.html |publisher=NASA/[[GSFC]]}}</ref> Oort was studying stellar motions in the [[Local Group|local galactic neighborhood]] and found the mass in the galactic plane must be greater than what was observed, but this measurement was later determined to be erroneous.<ref>{{cite journal |last1=Kuijken |first1=K. |last2=Gilmore |first2=G. |date=July 1989 |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=239 |issue=2 |pages=651–664 |bibcode=1989MNRAS.239..651K |title=The Mass Distribution in the Galactic Disc – Part III – the Local Volume Mass Density |doi=10.1093/mnras/239.2.651 }}</ref> In 1933, Swiss astrophysicist [[Fritz Zwicky]], who studied [[galaxy cluster]]s while working at the California Institute of Technology, made a similar inference.<ref name="zwicky1933">{{Cite journal |last=Zwicky |first=F. |author-link=Fritz Zwicky |date=1933 |title=Die Rotverschiebung von extragalaktischen Nebeln |trans-title=The red shift of extragalactic nebulae |journal=[[Helvetica Physica Acta]] |volume=6 |pages=110–127 |bibcode=1933AcHPh...6..110Z}} From p 125: ''"Um, wie beobachtet, einen mittleren Dopplereffekt von 1000 km/sek oder mehr zu erhalten, müsste also die mittlere Dichte im Comasystem mindestens 400 mal grösser sein als die auf Grund von Beobachtungen an leuchtender Materie abgeleitete. Falls sich dies bewahrheiten sollte, würde sich also das überraschende Resultat ergeben, dass dunkle Materie in sehr viel grösserer Dichte vorhanden ist als leuchtende Materie."'' (In order to obtain an average Doppler effect of 1000 km/s or more, as observed, the average density in the Coma system would thus have to be at least 400 times greater than that derived on the basis of observations of luminous matter. If this were to be confirmed, the surprising result would then follow that dark matter is present in very much greater density than luminous matter.) </ref><ref name="zwicky1937">{{Cite journal |last=Zwicky |first=F. |author-link=Fritz Zwicky |title=On the Masses of Nebulae and of Clusters of Nebulae |date=1937 |journal=[[The Astrophysical Journal]] |volume=86 |pages=217–246 |bibcode=1937ApJ....86..217Z |doi=10.1086/143864}}</ref> Zwicky applied the [[virial theorem]] to the [[Coma Cluster]] and obtained evidence of unseen mass he called ''dunkle Materie'' ('dark matter'). Zwicky estimated its mass based on the motions of galaxies near its edge and compared that to an estimate based on its brightness and number of galaxies. He estimated the cluster had about 400 times more mass than was visually observable. The gravity effect of the visible galaxies was far too small for such fast orbits, thus mass must be hidden from view. Based on these conclusions, Zwicky inferred some unseen matter provided the mass and associated gravitation attraction to hold the cluster together.<ref>Some details of Zwicky's calculation and of more modern values are given in {{Citation |first=M. |last=Richmond |title=Using the virial theorem: the mass of a cluster of galaxies |url=http://spiff.rit.edu/classes/phys440/lectures/gal_clus/gal_clus.html |access-date=10 July 2007}}</ref> Zwicky's estimates were off by more than an order of magnitude, mainly due to an obsolete value of the [[Hubble constant]];<ref>{{cite book |first=Katherine |last=Freese |title=The cosmic cocktail: Three parts dark matter |url={{google books |plainurl=y |id=c2B8AgAAQBAJ}} |year=2014 |publisher=Princeton University Press |isbn=978-1-4008-5007-5}}</ref> the same calculation today shows a smaller fraction, using greater values for luminous mass. Nonetheless, Zwicky did correctly conclude from his calculation that the bulk of the matter was dark.<ref name=":1"/> Further indications of [[mass-to-light ratio]] anomalies came from measurements of galaxy rotation curves. In 1939, [[Horace W. Babcock]] reported the rotation curve for the [[Andromeda Galaxy|Andromeda nebula]] (known now as the Andromeda Galaxy), which suggested the mass-to-luminosity ratio increases radially.<ref name=":0">{{cite journal |bibcode=1939LicOB..19...41B |doi=10.5479/ADS/bib/1939LicOB.19.41B |title=The rotation of the Andromeda Nebula |journal=Lick Observatory Bulletin|volume=19 |pages=41–51 |year=1939 |last1=Babcock |first1=Horace W. |doi-access=free }}</ref> He attributed it to either light absorption within the galaxy or modified dynamics in the outer portions of the spiral and not to the missing matter he had uncovered. Following [[Horace W. Babcock|Babcock's]] 1939 report of unexpectedly rapid rotation in the outskirts of the Andromeda galaxy and a mass-to-light ratio of 50; in 1940 [[Jan Oort]] discovered and wrote about the large non-visible halo of [[NGC 3115|NGC&nbsp;3115]].<ref>{{cite journal |last1=Oort |first1=J.H. |date=April 1940 |journal=The Astrophysical Journal|volume=91 |issue=3 |pages=273–306 |bibcode=1940ApJ....91..273O |title=Some problems concerning the structure and dynamics of the galactic system and the elliptical nebulae NGC&nbsp;3115 and 4494 |doi=10.1086/144167 |url=https://openaccess.leidenuniv.nl/bitstream/handle/1887/8533/008_653_032.pdf?sequence=1 |hdl=1887/8533 |hdl-access=free }}</ref> === 1960s === Early radio astronomy observations, performed by Seth Shostak, later SETI Institute Senior Astronomer, showed a half-dozen galaxies spun too fast in their outer regions - pointing to the existence of dark matter as a means of creating the gravitational pull needed to keep the stars in their orbits.<ref>{{cite web |url=https://astronomy.com/-/media/Files/PDF/Superstars/Shostak.pdf |archive-url=https://web.archive.org/web/20210927140634/https://www.astronomy.com/-/media/Files/PDF/Superstars/Shostak.pdf |archive-date=2021-09-27 |url-status=live |title=Superstars of Astronomy podcast |author=<!--Not stated--> |date= |website= |publisher= |access-date= |quote=}}</ref> === 1970s === [[Vera Rubin]], [[Kent Ford (astronomer)|Kent Ford]], and [[Ken Freeman (astronomer)|Ken Freeman]]'s work in the 1960s and 1970s<ref>{{cite journal |last1=Freeman |first1=K.C. |date=June 1970 |journal=The Astrophysical Journal |volume=160 |pages=811–830 |bibcode=1970ApJ...160..811F |title=On the Disks of Spiral and S0 Galaxies |doi=10.1086/150474 }}</ref> provided further strong evidence, also using galaxy rotation curves.<ref name="NYT-20161227">{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |title=Vera Rubin, 88, Dies; Opened Doors in Astronomy, and for Women |url=https://www.nytimes.com/2016/12/27/science/vera-rubin-astronomist-who-made-the-case-for-dark-matter-dies-at-88.html |date=27 December 2016 |newspaper=[[The New York Times]] |access-date=27 December 2016 }}</ref><ref>{{cite web |url=http://www.darkmatterphysics.com/Galactic-rotation-curves-of-spiral-galaxies.htm |title=First observational evidence of dark matter |archive-url=https://web.archive.org/web/20130625183052/http://www.darkmatterphysics.com/Galactic-rotation-curves-of-spiral-galaxies.htm |archive-date=25 June 2013 |website=Darkmatterphysics.com |access-date=6 August 2013}}</ref><ref name="Rubin1970" /> Rubin and Ford worked with a new [[spectrograph]] to measure the [[galaxy rotation curve|velocity curve]] of edge-on [[spiral galaxies]] with greater accuracy.<ref name="Rubin1970">{{cite journal |last1=Rubin |first1=Vera C. |author-link1=Vera Rubin |last2=Ford | first2=W. Kent Jr. |author-link2=Kent Ford (astronomer) |date=February 1970 |title=Rotation of the Andromeda Nebula from a Spectroscopic Survey of Emission Regions |journal=[[The Astrophysical Journal]]|volume=159 |pages=379–403 |bibcode=1970ApJ...159..379R |doi=10.1086/150317|s2cid=122756867 }}</ref> This result was confirmed in 1978.<ref>{{cite thesis |last=Bosma |first=A. |date=1978 |title=The distribution and kinematics of neutral hydrogen in spiral galaxies of various morphological types |url=http://nedwww.ipac.caltech.edu/level5/March05/Bosma/frames.html |type=PhD Thesis |publisher=[[Rijksuniversiteit Groningen]]}}</ref> An influential paper presented Rubin and Ford's results in 1980.<ref name="Rubin1980">{{Cite journal |last1=Rubin |first1=V. |last2=Thonnard |first2=W.K. Jr. |last3=Ford |first3=N. |date=1980 |title=Rotational Properties of 21 Sc Galaxies with a Large Range of Luminosities and Radii from NGC 4605 (''R'' = 4kpc) to UGC 2885 (''R'' = 122kpc) |journal=The Astrophysical Journal|volume=238 |page=471 |bibcode=1980ApJ...238..471R |doi=10.1086/158003}}</ref> They showed most galaxies must contain about six times as much dark as visible mass;{{sfn|Randall|2015|pp=13–14}} thus, by around 1980 the apparent need for dark matter was widely recognized as a major unsolved problem in astronomy.<ref name="NYT-20161227" /> At the same time Rubin and Ford were exploring optical rotation curves, radio astronomers were making use of new radio telescopes to map the 21&nbsp;cm line of atomic hydrogen in nearby galaxies. The radial distribution of interstellar atomic hydrogen ([[H I region|H-I]]) often extends to much larger galactic radii than those accessible by optical studies, extending the sampling of rotation curves – and thus of the total mass distribution – to a new dynamical regime. Early mapping of Andromeda with the 300&nbsp;foot telescope at [[Green Bank Observatory|Green Bank]]<ref name="Roberts1966">{{Cite journal |last1=Roberts |first1=Morton S. <!-- |author-link1=Morton S. Roberts (astronomer) --> |date=May 1966 |title=A High-Resolution 21&nbsp;cm hydrogen-line survey of the Andromeda nebula |journal=The Astrophysical Journal |volume=159 |pages=639–656 |bibcode=1966ApJ...144..639R |doi=10.1086/148645}}</ref> and the 250&nbsp;foot dish at [[Jodrell Bank Observatory|Jodrell Bank]]<ref name="Gottesman1966">{{Cite journal |last1=Gottesman |first1=S.T. <!-- |author-link1=S. T. Gottesman (astronomer) --> |last2=Davies |first2=R.D. |author-link2=Rod Davies |last3=Reddish |first3=V.C. |author-link3=Vincent Cartledge Reddish |date=1966 |title=A neutral hydrogen survey of the southern regions of the Andromeda nebula |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=133 |issue=4 |pages=359–387 |bibcode=1966MNRAS.133..359G|doi=10.1093/mnras/133.4.359|doi-access=free }}</ref> already showed the H-I rotation curve did not trace the expected Keplerian decline. As more sensitive receivers became available, Morton Roberts and Robert Whitehurst<ref name="Roberts1975">{{cite journal |last1=Roberts |first1=Morton S. <!-- |author-link1=Morton S. Roberts (astronomer) --> |last2=Whitehurst |first2=Robert N. <!-- |author-link2=Robert N. Whitehurst (astronomer) --> |date=October 1975 |title=The rotation curve and geometry of M&nbsp;31 at large galactocentric distances |journal=The Astrophysical Journal |volume=201 |pages=327–346 |bibcode=1975ApJ...201..327R |doi=10.1086/153889}}</ref> were able to trace the rotational velocity of Andromeda to 30 kpc, much beyond the optical measurements. Illustrating the advantage of tracing the gas disk at large radii, Figure&nbsp;16 of that paper<ref name="Roberts1975" /> combines the optical data<ref name="Rubin1970" /> (the cluster of points at radii of less than 15&nbsp;kpc with a single point further out) with the H-I data between 20–30&nbsp;kpc, exhibiting the flatness of the outer galaxy rotation curve; the solid curve peaking at the center is the optical surface density, while the other curve shows the cumulative mass, still rising linearly at the outermost measurement. In parallel, the use of interferometric arrays for extragalactic H-I spectroscopy was being developed. In 1972, David Rogstad and [[Seth Shostak]]<ref name="Rogstad1972">{{cite journal |last1=Rogstad |first1=D.H. <!-- |author-link1=David H. Rogstad (astronomer) --> |last2=Shostak |first2=G. Seth |author-link2=Seth Shostak |date=September 1972 |title=Gross properties of five Scd galaxies as determined from 21&nbsp;centimeter observations |journal=The Astrophysical Journal |volume=176 |pages=315–321 |bibcode=1972ApJ...176..315R |doi=10.1086/151636}}</ref> published H-I rotation curves of five spirals mapped with the Owens Valley interferometer; the rotation curves of all five were very flat, suggesting very large values of mass-to-light ratio in the outer parts of their extended H-I disks. A stream of observations in the 1980s supported the presence of dark matter, including [[gravitational lensing]] of background objects by [[galaxy cluster]]s,{{sfn|Randall|2015|pp=14–16}} the temperature distribution of hot gas in galaxies and clusters, and the pattern of [[anisotropy|anisotropies]] in the [[cosmic microwave background]]. According to consensus among cosmologists, dark matter is composed primarily of a not yet characterized type of [[subatomic particle]].<ref name="Copi 1995">{{cite journal |last1=Copi |first1=C.J. |last2=Schramm |first2=D.N. |last3=Turner |first3=M.S. |date=1995 |title=Big-Bang Nucleosynthesis and the Baryon Density of the Universe |journal=[[Science (journal)|Science]] |volume=267 |issue=5195 |pages=192–199 |arxiv=astro-ph/9407006 |bibcode=1995Sci...267..192C |doi=10.1126/science.7809624 |pmid=7809624|s2cid=15613185 |url=http://cds.cern.ch/record/265576 }}</ref><ref name="Bergstrom 2000">{{cite journal |last=Bergstrom |first=L. |year=2000 |title=Non-baryonic dark matter: Observational evidence and detection methods |journal=[[Reports on Progress in Physics]] |volume=63 |issue=5 |pages=793–841 |arxiv=hep-ph/0002126 |bibcode=2000RPPh...63..793B |doi=10.1088/0034-4885/63/5/2r3|s2cid=119349858 }}</ref> The search for this particle, by a variety of means, is one of the major efforts in [[particle physics]].<ref name="bertone hooper silk">{{cite journal |last1=Bertone |first1=G. |last2=Hooper |first2=D. |last3=Silk |first3=J. |year=2005 |title=Particle dark matter: Evidence, candidates and constraints |journal=[[Physics Reports]] |volume=405 |issue=5–6 |pages=279–390 |arxiv=hep-ph/0404175 |bibcode=2005PhR...405..279B |doi=10.1016/j.physrep.2004.08.031|s2cid=118979310 }}</ref> == Technical definition == {{see also|Friedmann equations}} In standard cosmology, matter is anything whose energy density scales with the inverse cube of the [[scale factor (cosmology)|scale factor]], i.e., {{nowrap|''ρ'' ∝ ''a''{{sup|−3}}.}} This is in contrast to radiation, which scales as the inverse fourth power of the scale factor {{nowrap|''ρ'' ∝ ''a''{{sup|−4}},}} and a [[cosmological constant]], which is independent of ''a''. The different scale factors for matter and radiation are a consequence of radiation [[redshift]]: for example, after gradually doubling the diameter of the observable Universe via cosmic expansion in General Relativity, ''a'' has been doubled. The energy of the [[cosmic background radiation]] has been halved (because the wavelength of each photon has doubled);<ref>{{cite news |last1=Siegel |first1=Ethan |title=Is Energy Conserved When Photons Redshift In Our Expanding Universe? |url=https://www.forbes.com/sites/startswithabang/2019/08/14/is-energy-conserved-when-photons-redshift-due-to-the-expanding-universe/?sh=745b3e3a3efa |access-date=5 November 2022 |work=[[Starts With a Bang]] |date=2019 |language=en}}</ref> the energy of ultra-relativistic particles, such as early-era standard-model neutrinos, is similarly halved. (However, in the modern cosmic era, this neutrino field has cooled and started to behave more like matter and less like radiation.) The cosmological constant, as an intrinsic property of space, has a constant energy density regardless of the volume under consideration.<ref>{{cite web |url=http://www.damtp.cam.ac.uk/user/db275/Cosmology/Lectures.pdf |title=Cosmology: Part&nbsp;III |department=Mathematical Tripos |publisher=Cambridge University |first=Daniel |last=Baumann |pages=21–22 |access-date=24 January 2017 |archive-url=https://web.archive.org/web/20170202065045/http://www.damtp.cam.ac.uk/user/db275/Cosmology/Lectures.pdf |archive-date=2 February 2017 |url-status=dead}}</ref><ref group=lower-alpha>[[Dark energy]] is a term often used nowadays as a substitute for cosmological constant. It is basically the same except that dark energy might depend on scale factor in some unknown way rather than necessarily being constant.</ref> In principle, "dark matter" means all components of the universe which are not visible but still obey {{nowrap|''ρ'' ∝ ''a''{{sup|−3}}.}} In practice, the term "dark matter" is often used to mean only the non-baryonic component of dark matter, i.e., excluding "[[missing baryon problem|missing baryons]]". Context will usually indicate which meaning is intended. == Observational evidence == [[File: Artist’s impression of the expected dark matter distribution around the Milky Way.ogv|thumb|This artist's impression shows the expected distribution of dark matter in the [[Milky Way]] galaxy as a blue halo of material surrounding the galaxy.<ref> {{cite press release |date=18 April 2012 |title=Serious Blow to Dark Matter Theories? |url=http://www.eso.org/public/news/eso1217/ |publisher=[[European Southern Observatory]]}}</ref>]] === Galaxy rotation curves === {{Main|Galaxy rotation curve}} [[File:GalacticRotation2.svg|thumb|[[Galaxy rotation curve|Rotation curve]] of a typical spiral galaxy: predicted ('''A''') and observed ('''B'''). Dark matter can explain the 'flat' appearance of the velocity curve out to a large radius.]]The arms of [[spiral galaxies]] rotate around the galactic center. The luminous mass density of a spiral galaxy decreases as one goes from the center to the outskirts. If luminous mass were all the matter, then we can model the galaxy as a point mass in the centre and test masses orbiting around it, similar to the [[Solar System]].<ref group=lower-alpha>This is a consequence of the [[shell theorem]] and the observation that spiral galaxies are spherically symmetric to a large extent (in 2D).</ref> From [[Kepler's Second Law]], it is expected that the rotation velocities will decrease with distance from the center, similar to the Solar System. This is not observed.<ref>{{cite journal |author1=Corbelli, E. |author2=Salucci, P. |title=The extended rotation curve and the dark matter halo of M33|journal=[[Monthly Notices of the Royal Astronomical Society]] |date=2000 |volume=311 |issue=2 |pages=441–447 |doi=10.1046/j.1365-8711.2000.03075.x |arxiv=astro-ph/9909252 |bibcode=2000MNRAS.311..441C|s2cid=10888599 }}</ref> Instead, the galaxy rotation curve remains flat as distance from the center increases. If Kepler's laws are correct, then the obvious way to resolve this discrepancy is to conclude the mass distribution in spiral galaxies is not similar to that of the Solar System. In particular, there is a lot of non-luminous matter (dark matter) in the outskirts of the galaxy. === Velocity dispersions === {{main|Velocity dispersion}} Stars in bound systems must obey the [[virial theorem]]. The theorem, together with the measured velocity distribution, can be used to measure the mass distribution in a bound system, such as elliptical galaxies or globular clusters. With some exceptions, velocity dispersion estimates of elliptical galaxies<ref>{{cite journal |last1=Faber |first1=S.M. |last2=Jackson |first2=R.E. |date=1976 |title=Velocity dispersions and mass-to-light ratios for elliptical galaxies |journal=The Astrophysical Journal |volume=204 |pages=668–683 |bibcode=1976ApJ...204..668F |doi=10.1086/154215}}</ref> do not match the predicted velocity dispersion from the observed mass distribution, even assuming complicated distributions of stellar orbits.<ref>{{cite book |last1=Binny |first1=James |last2=Merrifield |first2=Michael |year=1998 |title=Galactic Astronomy |publisher=Princeton University Press |pages=712–713}}</ref> As with galaxy rotation curves, the obvious way to resolve the discrepancy is to postulate the existence of non-luminous matter. === Galaxy clusters === [[Galaxy clusters]] are particularly important for dark matter studies since their masses can be estimated in three independent ways: * From the scatter in radial velocities of the galaxies within clusters * From [[X-ray]]s emitted by hot gas in the clusters. From the X-ray energy spectrum and flux, the gas temperature and density can be estimated, hence giving the pressure; assuming pressure and gravity balance determines the cluster's mass profile. * [[Gravitational lens]]ing (usually of more distant galaxies) can measure cluster masses without relying on observations of dynamics (e.g., velocity). Generally, these three methods are in reasonable agreement that dark matter outweighs visible matter by approximately 5 to 1.<ref>{{cite journal |title=Cosmological Parameters from Clusters of Galaxies |journal=[[Annual Review of Astronomy and Astrophysics]] |volume=49 |issue=1 |pages=409–470 |doi=10.1146/annurev-astro-081710-102514 |arxiv=1103.4829 |bibcode=2011ARA&A..49..409A |year=2011 |last1=Allen |first1=Steven W. |last2=Evrard |first2=August E. |last3=Mantz |first3=Adam B. |s2cid=54922695 }}</ref> === Gravitational lensing === [[File:Gravitationell-lins-4.jpg|left|thumb|upright|Strong gravitational lensing as observed by the [[Hubble Space Telescope]] in [[Abell 1689]] indicates the presence of dark matter{{snd}}enlarge the image to see the lensing arcs.]] [[File:Comparison of rotating disc galaxies in the distant Universe and the present day.webm|thumb|Models of rotating disc galaxies in the present day (left) and ten&nbsp;billion years ago (right). In the present-day galaxy, dark matter{{snd}}shown in red{{snd}}is more concentrated near the center and it rotates more rapidly (effect exaggerated).]] [[File:Dark matter map of KiDS survey region (region G12).jpg|right|thumb|Dark matter map for a patch of sky based on gravitational lensing analysis of a Kilo-Degree survey.<ref>{{cite web |title=Dark matter may be smoother than expected – Careful study of large area of sky imaged by VST reveals intriguing result |url=https://www.eso.org/public/news/eso1642/ |website=www.eso.org |access-date=8 December 2016}}</ref>]] One of the consequences of [[general relativity]] is massive objects (such as a [[galaxy cluster|cluster of galaxies]]) lying between a more distant source (such as a [[quasar]]) and an observer should act as a lens to [[Gravitational lens|bend]] light from this source. The more massive an object, the more lensing is observed. Strong lensing is the observed distortion of background galaxies into arcs when their light passes through such a gravitational lens. It has been observed around many distant clusters including [[Abell 1689]].<ref>{{cite journal |last1=Taylor |first1=A.N. |display-authors=etal |date=1998 |title=Gravitational lens magnification and the mass of Abell&nbsp;1689 |journal=The Astrophysical Journal |volume=501 |issue=2 |pages=539–553 |arxiv=astro-ph/9801158 |bibcode=1998ApJ...501..539T |doi=10.1086/305827|s2cid=14446661 }}</ref> By measuring the distortion geometry, the mass of the intervening cluster can be obtained. In the dozens of cases where this has been done, the mass-to-light ratios obtained correspond to the dynamical dark matter measurements of clusters.<ref>{{Cite journal |last1=Wu |first1=X. |last2=Chiueh |first2=T. |last3=Fang |first3=L. |last4=Xue |first4=Y. |date=1998 |title=A comparison of different cluster mass estimates: consistency or discrepancy? |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=301 |issue=3 |pages=861–871 |arxiv=astro-ph/9808179 |bibcode=1998MNRAS.301..861W |doi=10.1046/j.1365-8711.1998.02055.x |citeseerx=10.1.1.256.8523|s2cid=1291475 }}</ref> Lensing can lead to multiple copies of an image. By analyzing the distribution of multiple image copies, scientists have been able to deduce and map the distribution of dark matter around the [[MACS J0416.1-2403]] galaxy cluster.<ref>{{cite journal |last1=Cho |first1=Adrian |title=Scientists unveil the most detailed map of dark matter to date |journal=Science |year=2017 |doi=10.1126/science.aal0847 |url=https://www.science.org/content/article/scientists-unveil-most-detailed-map-dark-matter-date}}</ref><ref>{{cite journal |last1=Natarajan |first1=Priyamvada |last2=Chadayammuri |first2=Urmila |last3=Jauzac |first3=Mathilde |last4=Richard |first4=Johan |last5=Kneib |first5=Jean-Paul |last6=Ebeling |first6=Harald |last7=Jiang |first7=Fangzhou |last8=van den Bosch |first8=Frank |last9=Limousin |first9=Marceau |last10=Jullo |first10=Eric |last11=Atek |first11=Hakim |last12=Pillepich |first12=Annalisa |last13=Popa |first13=Cristina |last14=Marinacci |first14=Federico |last15=Hernquist |first15=Lars |last16=Meneghetti |first16=Massimo |last17=Vogelsberger |first17=Mark |display-authors=6 |title=Mapping substructure in the HST Frontier Fields cluster lenses and in cosmological simulations |journal=Monthly Notices of the Royal Astronomical Society |volume=468 |issue=2 |page=1962 |date=2017 |doi=10.1093/mnras/stw3385 |arxiv=1702.04348 |url=http://dro.dur.ac.uk/22700/1/22700.pdf |archive-url=https://web.archive.org/web/20180723070641/http://dro.dur.ac.uk/22700/1/22700.pdf |archive-date=2018-07-23 |url-status=live |bibcode=2017MNRAS.468.1962N|s2cid=113404396 }}</ref> [[Weak gravitational lensing]] investigates minute distortions of galaxies, using statistical analyses from vast [[redshift survey|galaxy surveys]]. By examining the apparent shear deformation of the adjacent background galaxies, the mean distribution of dark matter can be characterized. The mass-to-light ratios correspond to dark matter densities predicted by other large-scale structure measurements.<ref>{{cite journal |last1=Refregier |first1=A. |year=2003 |title=Weak gravitational lensing by large-scale structure |journal=[[Annual Review of Astronomy and Astrophysics]] |volume=41 |issue=1 |pages=645–668 |arxiv=astro-ph/0307212 |bibcode=2003ARA&A..41..645R |doi=10.1146/annurev.astro.41.111302.102207|s2cid=34450722 }}</ref> Dark matter does not bend light itself; mass (in this case the mass of the dark matter) bends [[spacetime]]. Light follows the curvature of spacetime, resulting in the lensing effect.<ref>{{cite web |url=https://www.learner.org/courses/physics/unit/text.html?unit=10&secNum=4 |title=Quasars, lensing, and dark matter |series=Physics for the 21st Century |publisher=Annenberg Foundation |year=2017 | url-status=dead | archive-url=https://web.archive.org/web/20130729023035/https://www.learner.org/courses/physics/unit/text.html?unit=10&secNum=4 | archive-date=July 29, 2013 }}</ref><ref>{{cite news |url=https://www.theregister.co.uk/2011/10/14/hubble_images_gravitational_lensing/ |title=Hubble snaps dark matter warping spacetime |author=Myslewski, Rik |newspaper=The Register |location=UK |date=14 October 2011}}</ref> In May 2021, a new detailed dark matter map was revealed by the [[Dark Energy Survey]] Collaboration.<ref>{{cite web |url=https://www.bbc.com/news/science-environment-57244708 |title=New dark matter map reveals cosmic mystery |website=BBC |date=28 May 2021 }}</ref> In addition, the map revealed previously undiscovered [[Galaxy filament|filamentary]] structures connecting galaxies, by using a [[machine learning]] method.<ref>{{cite news<!--|authors=Sungwook E. Hong, Donghui Jeong, Ho Seong Hwang, and Juhan Kim-->|author=Sungwook E. Hong|display-authors=etal|title=Revealing the Local Cosmic Web from Galaxies by Deep Learning|publisher=The Astrophysical Journal|year=2021|volume=913|issue=1|doi=10.3847/1538-4357/abf040}}</ref> === Cosmic microwave background === {{Main|Cosmic microwave background}} Although both dark matter and ordinary matter are matter, they do not behave in the same way. In particular, in the early universe, ordinary matter was ionized and interacted strongly with radiation via [[Thomson scattering]]. Dark matter does not interact directly with radiation, but it does affect the cosmic microwave background (CMB) by its gravitational potential (mainly on large scales) and by its effects on the density and velocity of ordinary matter. Ordinary and dark matter perturbations, therefore, evolve differently with time and leave different imprints on the CMB. The cosmic microwave background is very close to a perfect blackbody but contains very small temperature anisotropies of a few parts in 100,000. A sky map of anisotropies can be decomposed into an angular power spectrum, which is observed to contain a series of acoustic peaks at near-equal spacing but different heights. The series of peaks can be predicted for any assumed set of cosmological parameters by modern computer codes such as [[CMBFAST]] and [[CAMB]], and matching theory to data, therefore, constrains cosmological parameters.<ref name="Wayne Hu">The details are technical. For an intermediate-level introduction, see {{cite web |author=Hu, Wayne |title=Intermediate Guide to the Acoustic Peaks and Polarization |url=http://background.uchicago.edu/~whu/intermediate/intermediate.html |year=2001}}</ref> The first peak mostly shows the density of baryonic matter, while the third peak relates mostly to the density of dark matter, measuring the density of matter and the density of atoms.<ref name="Wayne Hu" /> The CMB anisotropy was first discovered by [[Cosmic Background Explorer|COBE]] in 1992, though this had too coarse resolution to detect the acoustic peaks. After the discovery of the first acoustic peak by the balloon-borne [[BOOMERanG]] experiment in 2000, the power spectrum was precisely observed by [[WMAP]] in 2003–2012, and even more precisely by the [[Planck spacecraft|''Planck'' spacecraft]] in 2013–2015. The results support the Lambda-CDM model.<ref name="Hinshaw2009">{{Cite journal |last1=Hinshaw |first1=G. |display-authors=etal |year=2009 |title=Five-year Wilkinson microwave anisotropy probe (WMAP) observations: Data processing, sky maps, and basic results |journal=The Astrophysical Journal Supplement |volume=180 |issue=2 |pages=225–245 |arxiv=0803.0732 |bibcode=2009ApJS..180..225H |doi=10.1088/0067-0049/180/2/225|s2cid=3629998 }}</ref><ref name="Planck15" /> The observed CMB angular power spectrum provides powerful evidence in support of dark matter, as its precise structure is well fitted by the [[Lambda-CDM model]],<ref name="Planck15"/> but difficult to reproduce with any competing model such as [[modified Newtonian dynamics]] (MOND).<ref name="Planck15">{{cite journal |last1=Ade |first1=P.A.R. |display-authors=etal |title=Planck 2015 results. XIII. Cosmological parameters |journal=Astron. Astrophys. |year=2016 |volume=594 |issue=13 |page=A13 |doi=10.1051/0004-6361/201525830 |bibcode=2016A&A...594A..13P |arxiv=1502.01589 |s2cid=119262962 }}</ref><ref>{{cite journal |last1=Skordis |first1=C. |display-authors=etal |title=Large scale structure in Bekenstein's theory of relativistic modified Newtonian dynamics |journal=Phys. Rev. Lett. |year=2006 |volume=96 |issue=1 |page=011301 |doi=10.1103/PhysRevLett.96.011301 |pmid=16486433 |bibcode=2006PhRvL..96a1301S |arxiv=astro-ph/0505519|s2cid=46508316 }}</ref> === Structure formation === {{Main|Structure formation}} [[File:COSMOS 3D dark matter map.png|thumb|alt=Mass map|3-D map of the large-scale distribution of dark matter, reconstructed from measurements of [[weak gravitational lensing]] with the [[Hubble Space Telescope]].<ref>{{Cite press release |date=7 January 2007 |title=Hubble Maps the Cosmic Web of "Clumpy" Dark Matter in 3-D |url=http://hubblesite.org/newscenter/archive/releases/2007/01/image/a/grav |publisher=NASA|url-status=dead | archive-url=https://web.archive.org/web/20130521012704/http://hubblesite.org/newscenter/archive/releases/2007/01/image/a/grav/ | archive-date=May 21, 2013}}</ref>]] Structure formation refers to the period after the Big Bang when density perturbations collapsed to form stars, galaxies, and clusters. Prior to structure formation, the [[FRW metric|Friedmann solutions]] to general relativity describe a homogeneous universe. Later, small anisotropies gradually grew and condensed the homogeneous universe into stars, galaxies and larger structures. Ordinary matter is affected by radiation, which is the dominant element of the universe at very early times. As a result, its density perturbations are washed out and unable to condense into structure.<ref name="Jaffe">{{cite web |author=Jaffe, A.H. |title=Cosmology 2012: Lecture Notes |url=http://astro.imperial.ac.uk/sites/default/files/cosmology.pdf| url-status=dead | archive-url=https://web.archive.org/web/20160717223916/http://astro.imperial.ac.uk/sites/default/files/cosmology.pdf | archive-date=July 17, 2016}}</ref> If there were only ordinary matter in the universe, there would not have been enough time for density perturbations to grow into the galaxies and clusters currently seen. Dark matter provides a solution to this problem because it is unaffected by radiation. Therefore, its density perturbations can grow first. The resulting gravitational potential acts as an attractive [[potential well]] for ordinary matter collapsing later, speeding up the structure formation process.<ref name="Jaffe" /><ref>{{cite journal |title=Constraints on the composite photon theory |author=Low, L.F. |doi=10.1142/S021773231675002X |journal=Modern Physics Letters A |volume=31 |issue=36 |pages=1675002 |date=12 October 2016 |bibcode=2016MPLA...3175002L|url=https://zenodo.org/record/896052 }}</ref> === Bullet Cluster === {{Main|Bullet Cluster}} If dark matter does not exist, then the next most likely explanation must be that general relativity – the prevailing theory of gravity – is incorrect and should be modified. The Bullet Cluster, the result of a recent collision of two galaxy clusters, provides a challenge for modified gravity theories because its apparent center of mass is far displaced from the baryonic center of mass.<ref>{{cite journal |last1=Clowe |first1=Douglas |display-authors=etal |year=2006 |title=A Direct Empirical Proof of the Existence of Dark Matter |journal=The Astrophysical Journal Letters |volume=648 |issue=2 |pages=L109–L113 |arxiv=astro-ph/0608407 |doi=10.1086/508162 |bibcode=2006ApJ...648L.109C|s2cid=2897407 }}</ref> Standard dark matter models can easily explain this observation, but modified gravity has a much harder time,<ref>{{cite web |url=https://arstechnica.com/science/2017/09/science-in-progress-did-the-bullet-cluster-withstand-scrutiny/ |title=Science-in-progress: Did the Bullet Cluster withstand scrutiny? |website=Ars Technica |author=Lee, Chris |date=21 September 2017}}</ref><ref>{{cite magazine |url=https://www.forbes.com/sites/startswithabang/2017/11/09/the-bullet-cluster-proves-dark-matter-exists-but-not-for-the-reason-most-physicists-think/#3032b6081738 |title=The Bullet Cluster proves dark matter exists, but not for the reason most physicists think |magazine=Forbes |author=Siegel, Ethan |author-link=Ethan Siegel |date=9 November 2017}}</ref> especially since the observational evidence is model-independent.<ref>{{cite conference |url=http://cosis.net/abstracts/COSPAR2006/02655/COSPAR2006-A-02655.pdf |archive-url=https://web.archive.org/web/20060821074820/http://www.cosis.net/abstracts/COSPAR2006/02655/COSPAR2006-A-02655.pdf |archive-date=2006-08-21 |url-status=live |title=Dark matter and the Bullet Cluster |author1=Markevitch, M. |author2=Randall, S. |author3=Clowe, D. |author4=Gonzalez, A. |author5=Bradac, M. |name-list-style=amp |conference=36th COSPAR Scientific Assembly |date=16–23 July 2006 |location=Beijing, China}} Abstract only</ref> === Type Ia supernova distance measurements === {{Main|Type Ia supernova|Shape of the universe}} Type Ia [[supernovae]] can be used as [[standard candles]] to measure extragalactic distances, which can in turn be used to measure how fast the universe has expanded in the past.<ref>{{cite journal |last1=Planck Collaboration |last2=Aghanim |first2=N. |last3=Akrami |first3=Y. |last4=Ashdown |first4=M. |last5=Aumont |first5=J. |last6=Baccigalupi |first6=C. |last7=Ballardini |first7=M. |last8=Banday |first8=A. J. |last9=Barreiro |first9=R. B. |last10=Bartolo |first10=N. |last11=Basak |first11=S. |title=Planck 2018 results. VI. Cosmological parameters |journal=Astronomy & Astrophysics |year=2020 |volume=641 |pages=A6 |doi=10.1051/0004-6361/201833910 |arxiv=1807.06209 |bibcode=2020A&A...641A...6P |s2cid=119335614}}</ref> Data indicates the universe is expanding at an accelerating rate, the cause of which is usually ascribed to [[dark energy]].<ref>{{Cite journal |last1=Kowalski |first1=M. |display-authors=etal |year=2008 |title=Improved Cosmological Constraints from New, Old, and Combined Supernova Data Sets |journal=The Astrophysical Journal |volume=686 |issue=2 |pages=749–778 |arxiv=0804.4142 |bibcode=2008ApJ...686..749K |doi=10.1086/589937 |s2cid=119197696 }}</ref> Since observations indicate the universe is almost flat,<ref name="NASA_Shape">{{cite web|url= https://map.gsfc.nasa.gov/universe/uni_shape.html |title=Will the Universe expand forever? |publisher=NASA |date=24 January 2014 |access-date=2021-03-28}}</ref><ref name="Fermi_Flat">{{cite web|url= https://www.symmetrymagazine.org/article/april-2015/our-flat-universe |title=Our flat universe |publisher=FermiLab/SLAC |date=7 April 2015 |access-date=2021-03-28}}</ref><ref>{{cite journal |title=Unexpected connections |first=Marcus Y. |last=Yoo |journal=Engineering & Science |volume=74 |issue=1 |date=2011 |page=30}}</ref> it is expected the total energy density of everything in the universe should sum to 1 ({{nowrap|Ω_tot ≈ 1}}). The measured dark energy density is {{nowrap|Ω_Λ ≈ 0.690}}; the observed ordinary (baryonic) matter energy density is {{nowrap|Ω_b ≈ 0.0482}} and the energy density of radiation is negligible. This leaves a missing {{nowrap|Ω_dm ≈ 0.258}} which nonetheless behaves like matter (see technical definition section above){{snd}} dark matter.<ref name="planckesa2015">{{cite web |url=http://www.cosmos.esa.int/web/planck/publications |title=Planck Publications: Planck 2015 Results |publisher=European Space Agency |date=February 2015 |access-date=9 February 2015}}</ref> === Sky surveys and baryon acoustic oscillations === {{Main|Baryon acoustic oscillations}} Baryon acoustic oscillations (BAO) are fluctuations in the density of the visible baryonic matter (normal matter) of the universe on large scales. These are predicted to arise in the Lambda-CDM model due to acoustic oscillations in the photon–baryon fluid of the early universe, and can be observed in the cosmic microwave background angular power spectrum. BAOs set up a preferred length scale for baryons. As the dark matter and baryons clumped together after recombination, the effect is much weaker in the galaxy distribution in the nearby universe, but is detectable as a subtle (≈1&nbsp;percent) preference for pairs of galaxies to be separated by 147&nbsp;Mpc, compared to those separated by 130–160&nbsp;Mpc. This feature was predicted theoretically in the 1990s and then discovered in 2005, in two large galaxy redshift surveys, the [[Sloan Digital Sky Survey]] and the [[2dF Galaxy Redshift Survey]].<ref> {{Cite journal |last1=Percival |first1=W.J. |display-authors=etal |year=2007 |title=Measuring the Baryon Acoustic Oscillation scale using the Sloan Digital Sky Survey and 2dF Galaxy Redshift Survey |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=381 |issue=3 |pages=1053–1066 |arxiv=0705.3323 |bibcode=2007MNRAS.381.1053P |doi=10.1111/j.1365-2966.2007.12268.x}}</ref> Combining the CMB observations with BAO measurements from galaxy [[redshift survey]]s provides a precise estimate of the [[Hubble's law|Hubble constant]] and the average matter density in the Universe.<ref name="Komatsu2009">{{Cite journal |last1=Komatsu |first1=E. |display-authors=etal |year=2009 |title=Five-Year Wilkinson Microwave Anisotropy Probe Observations: Cosmological Interpretation |journal=The Astrophysical Journal Supplement|volume=180 |issue=2 |pages=330–376 |arxiv=0803.0547 |bibcode=2009ApJS..180..330K |doi=10.1088/0067-0049/180/2/330|s2cid=119290314 }}</ref> The results support the Lambda-CDM model. === Redshift-space distortions === Large galaxy [[redshift survey]]s may be used to make a three-dimensional map of the galaxy distribution. These maps are slightly distorted because distances are estimated from observed [[redshift]]s; the redshift contains a contribution from the galaxy's so-called peculiar velocity in addition to the dominant Hubble expansion term. On average, superclusters are expanding more slowly than the cosmic mean due to their gravity, while voids are expanding faster than average. In a redshift map, galaxies in front of a supercluster have excess radial velocities towards it and have redshifts slightly higher than their distance would imply, while galaxies behind the supercluster have redshifts slightly low for their distance. This effect causes superclusters to appear squashed in the radial direction, and likewise voids are stretched. Their angular positions are unaffected. This effect is not detectable for any one structure since the true shape is not known, but can be measured by averaging over many structures. It was predicted quantitatively by Nick Kaiser in 1987, and first decisively measured in 2001 by the [[2dF Galaxy Redshift Survey]].<ref>{{cite journal |last1=Peacock |first1=J. |display-authors=etal |title=A measurement of the cosmological mass density from clustering in the 2dF Galaxy Redshift Survey |journal=Nature |year=2001 |volume=410 |issue=6825 |pages=169–173 |arxiv=astro-ph/0103143 |bibcode=2001Natur.410..169P |doi=10.1038/35065528 |pmid=11242069|s2cid=1546652 }}</ref> Results are in agreement with the [[Lambda-CDM model]]. === Lyman-alpha forest === {{Main|Lyman-alpha forest}} In [[astronomical spectroscopy]], the Lyman-alpha forest is the sum of the [[Spectral line|absorption lines]] arising from the [[Lyman series|Lyman-alpha]] transition of [[Hydrogen line|neutral hydrogen]] in the spectra of distant [[Galaxy|galaxies]] and [[quasar]]s. Lyman-alpha forest observations can also constrain cosmological models.<ref>{{Cite journal |last1=Viel |first1=M. |last2=Bolton |first2=J.S. |last3=Haehnelt |first3=M.G. |date=2009 |title=Cosmological and astrophysical constraints from the Lyman α forest flux probability distribution function |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=399 |issue=1 |pages=L39–L43 |arxiv=0907.2927 |bibcode=2009MNRAS.399L..39V |doi=10.1111/j.1745-3933.2009.00720.x|s2cid=12470622 }}</ref> These constraints agree with those obtained from WMAP data. ==Theoretical classifications== === Composition === There are various [[hypotheses]] about what dark matter could consist of, as set out in the table below. {{unsolved|physics|What is dark matter? How was it generated?}} {| class = wikitable |+ Some dark matter hypotheses<ref>{{cite web |url=https://phys.org/news/2018-10-era-quest-dark.html |title=A new era in the quest for dark matter |publisher=Phys.org |author=University of Amsterdam}}</ref> |- |rowspan=3| Light bosons | [[quantum chromodynamics]] [[axion]]s |- | [[WISP (particle physics)|axion-like particle]]s |- | [[fuzzy cold dark matter]] |- |rowspan=2| neutrinos | [[neutrino|Standard Model]] |- | [[sterile neutrinos]] |- |rowspan=5| [[weak scale]] | [[supersymmetry]] |- | [[extra dimensions]] |- | [[little Higgs]] |- | [[effective field theory]] |- |simplified models |- |rowspan=3| other particles | [[Weakly interacting massive particles]] |- | [[self-interacting dark matter]] |- | [[strangelet]] |- | [[superfluid vacuum theory]] |- | [[Dynamical Dark Matter]] |- |rowspan=3| macroscopic | [[primordial black hole]]s<ref>{{cite journal |last1=Espinosa |first1=J. R. |last2=Racco |first2=D. |last3=Riotto |first3=A. |title=A Cosmological Signature of the Standard Model Higgs Vacuum Instability: Primordial Black Holes as Dark Matter |journal=Physical Review Letters |volume=120 |issue=12 |pages=121301 |doi=10.1103/PhysRevLett.120.121301 |pmid=29694085 |date=23 March 2018|arxiv=1710.11196 |bibcode=2018PhRvL.120l1301E |s2cid=206309027 }}</ref><ref>{{cite journal |last1=Clesse |first1=Sebastien |last2=García-Bellido |first2=Juan |title=Seven Hints for Primordial Black Hole Dark Matter |journal=Physics of the Dark Universe |volume=22 |pages=137–146 |arxiv=1711.10458 |bibcode=2018PDU....22..137C |doi=10.1016/j.dark.2018.08.004 |year=2018 |s2cid=54594536 }}</ref><ref>{{cite journal |last1=Lacki |first1=Brian C. |last2=Beacom |first2=John F. |title=Primordial Black Holes as Dark Matter: Almost All or Almost Nothing |journal=The Astrophysical Journal |date=12 August 2010 |volume=720 |issue=1 |pages=L67–L71 |doi=10.1088/2041-8205/720/1/L67 |language=en |issn=2041-8205 |arxiv=1003.3466 |bibcode=2010ApJ...720L..67L |s2cid=118418220 }}</ref><ref>{{cite journal |last1=Kashlinsky |first1=A. |title=LIGO gravitational wave detection, primordial black holes and the near-IR cosmic infrared background anisotropies |journal=The Astrophysical Journal |date=23 May 2016 |volume=823 |issue=2 |pages=L25 |doi=10.3847/2041-8205/823/2/L25 |issn=2041-8213|arxiv=1605.04023 |bibcode=2016ApJ...823L..25K |s2cid=118491150 }}</ref><ref>{{cite journal |last1=Frampton |first1=Paul H. |last2=Kawasaki |first2=Masahiro |last3=Takahashi |first3=Fuminobu |last4=Yanagida |first4=Tsutomu T. |title=Primordial Black Holes as All Dark Matter |journal=Journal of Cosmology and Astroparticle Physics |date=22 April 2010 |volume=2010 |issue=4 |pages=023 |doi=10.1088/1475-7516/2010/04/023 |issn=1475-7516|arxiv=1001.2308 |bibcode=2010JCAP...04..023F |s2cid=119256778 }}</ref><ref>{{cite journal |last1=Carneiro |first1=S. |last2=de Holanda |first2=P.C. |last3=Saa |first3=A. |title=Neutrino primordial Planckian black holes |journal=Physics Letters |date=2021 |volume=B822 |pages=136670 |doi=10.1016/j.physletb.2021.136670 |issn=0370-2693|bibcode=2021PhLB..82236670C |s2cid=244196281 }}</ref> |- | [[massive compact halo objects]] (MACHOs) |- | [[Macroscopic dark matter]] (Macros) |- |rowspan=3| [[modified gravity]] (MOG) | [[modified Newtonian dynamics]] (MoND) |- | [[Tensor–vector–scalar gravity]] (TeVeS) |- | [[Entropic gravity]] |} Dark matter can refer to any substance which interacts predominantly via gravity with visible matter (e.g., stars and planets). Hence in principle it need not be composed of a new type of fundamental particle but could, at least in part, be made up of standard baryonic matter, such as protons or neutrons. [[File:Fermi Observations of Dwarf Galaxies Provide New Insights on Dark Matter.ogv|thumb|[[Fermi Gamma-ray Space Telescope|Fermi-LAT]] observations of dwarf galaxies provide new insights on dark matter.]] ==== Baryonic matter ==== {{Distinguish|Missing baryon problem}} Most of the ordinary matter familiar to astronomers, including planets, brown dwarfs, red dwarfs, visible stars, white dwarfs, neutron stars, and black holes, is called ''baryonic matter'' (referring to the baryons that dominate the mass of most ordinary matter).<ref>{{cite journal |last1=Bertone |first1=Gianfranco |last2=Hooper |first2=Dan |title=History of dark matter |journal=Reviews of Modern Physics |date=15 October 2018 |volume=90 |issue=4 |pages=045002 |doi=10.1103/RevModPhys.90.045002|arxiv=1605.04909 |bibcode=2018RvMP...90d5002B |s2cid=18596513 }}</ref><ref name=BaryonicSource01>{{cite web |url=http://astronomy.swin.edu.au/cosmos/B/Baryonic+Matter |title=Baryonic Matter |website=COSMOS – The SAO Encyclopedia of Astronomy |publisher=[[Swinburne University of Technology]]|access-date=16 November 2022}}</ref> Solitary [[black hole]]s, [[neutron star]]s, burnt-out dwarfs, and other massive objects that that are hard to detect are collectively known as MACHOs; some scientists initially hoped that baryonic MACHOs could account for and explain all the dark matter.{{sfn|Randall|2015|p=286}}<ref>{{cite news |title=MACHOs may be out of the running as a dark matter candidate |url=https://astronomy.com/news/2016/08/machos-may-be-out-of-the-running-as-a-dark-matter-candidate |access-date=16 November 2022 |work=Astronomy.com |date=2016 |language=en}}</ref> However, multiple lines of evidence suggest the majority of dark matter is not baryonic: * Sufficient diffuse, baryonic gas or dust would be visible when backlit by stars. * The theory of [[Big Bang nucleosynthesis]] predicts the observed [[abundance of the chemical elements]]. If there are more baryons, then there should also be more helium, lithium and heavier elements synthesized during the Big Bang.<ref>{{cite book |author=Weiss, Achim |url=http://www.einstein-online.info/spotlights/BBN |title=Big bang nucleosynthesis: Cooking up the first light elements |archive-url=https://web.archive.org/web/20130206021217/http://www.einstein-online.info/spotlights/BBN |archive-date=6 February 2013 |website=Einstein Online |volume=2 |year=2006 |page=1017}}</ref><ref>{{cite book |last1=Raine |first1=D. |last2=Thomas |first2=T. |date=2001 |title=An Introduction to the Science of Cosmology |page=30 |publisher=[[IOP Publishing]] |isbn=978-0-7503-0405-4 |oclc=864166846 }}</ref> Agreement with observed abundances requires that baryonic matter makes up between 4–5% of the universe's [[Friedmann equations#Density parameter|critical density]]. In contrast, [[large-scale structure of the universe|large-scale structure]] and other observations indicate that the total matter density is about 30% of the critical density.<ref name="planckesa2015" /> * Astronomical searches for [[gravitational microlensing]] in the [[Milky Way]] found at most only a small fraction of the dark matter may be in dark, compact, conventional objects (MACHOs, etc.); the excluded range of object masses is from half the Earth's mass up to 30&nbsp;solar masses, which covers nearly all the plausible candidates.<ref>{{cite journal |last1=Tisserand |first1=P. |last2=Le&nbsp;Guillou |first2=L. |last3=Afonso |first3=C. |last4=Albert |first4=J.N. |last5=Andersen |first5=J. |last6=Ansari |first6=R. |last7=Aubourg |first7=É. |last8=Bareyre |first8=P. |last9=Beaulieu |first9=J.P. |last10=Charlot |first10=X. |last11=Coutures |first11=C. |last12=Ferlet |first12=R. |last13=Fouqué |first13=P. |last14=Glicenstein |first14=J.F. |last15=Goldman |first15=B. |last16=Gould |first16=A. |last17=Graff |first17=D. |last18=Gros |first18=M. |last19=Haissinski |first19=J. |last20=Hamadache |first20=C. |last21=De Kat |first21=J. |last22=Lasserre |first22=T. |last23=Lesquoy |first23=É. |last24=Loup |first24=C. |last25=Magneville |first25=C. |last26=Marquette |first26=J.B. |last27=Maurice |first27=É. |last28=Maury |first28=A. |last29=Milsztajn |first29=A. |last30=Moniez |first30=M. |display-authors=6 |title=Limits on the Macho content of the Galactic Halo from the EROS-2 Survey of the Magellanic Clouds |journal=Astronomy and Astrophysics |volume=469 |issue=2 |pages=387–404 |year=2007 |doi=10.1051/0004-6361:20066017 |url=https://www.researchgate.net/publication/41714676 |arxiv=astro-ph/0607207 |bibcode=2007A&A...469..387T|s2cid=15389106 }}</ref><ref>{{Cite journal |last1=Graff |first1=D.S. |last2=Freese |first2=K. |title=Analysis of a ''Hubble Space Telescope'' Search for Red Dwarfs: Limits on Baryonic Matter in the Galactic Halo |journal=The Astrophysical Journal |volume=456 |issue=1996 |page=L49 |year=1996 |arxiv=astro-ph/9507097 |bibcode=1996ApJ...456L..49G |doi=10.1086/309850|s2cid=119417172 }}</ref><ref>{{cite journal |last1=Najita |first1=J.R. |last2=Tiede |first2=G.P. |last3=Carr |first3=J.S. |doi=10.1086/309477 |title=From Stars to Superplanets: The Low‐Mass Initial Mass Function in the Young Cluster IC 348 |journal=The Astrophysical Journal |volume=541 |issue=2 |pages=977–1003 |year=2000 |arxiv=astro-ph/0005290 |bibcode=2000ApJ...541..977N|s2cid=55757804 }}</ref><ref>{{cite journal |arxiv=1106.2925 |title=The OGLE View of Microlensing towards the Magellanic Clouds. IV. OGLE-III SMC Data and Final Conclusions on MACHOs |journal=Monthly Notices of the Royal Astronomical Society |volume=416 |issue=4 |pages=2949–2961 |last1=Wyrzykowski |first1=L. |last2=Skowron |first2=J. |last3=Kozlowski |first3=S. |last4=Udalski |first4=A. |last5=Szymanski |first5=M.K. |last6=Kubiak |first6=M. |last7=Pietrzynski |first7=G. |last8=Soszynski |first8=I. |last9=Szewczyk |first9=O. |last10=Ulaczyk |first10=K. |last11=Poleski |first11=R. |last12=Tisserand |first12=P. |display-authors=6 |doi=10.1111/j.1365-2966.2011.19243.x |year=2011 |bibcode=2011MNRAS.416.2949W|s2cid=118660865 }}</ref><ref>{{cite arXiv |title=Death of stellar baryonic dark matter candidates |first1=Katherine |last1=Freese |eprint=astro-ph/0007444 |last2=Fields |first2=Brian |last3=Graff |first3=David |year=2000}}</ref><ref>{{cite book |journal=The First Stars |pages=4–6 |first1=Katherine |last1=Freese |arxiv=astro-ph/0002058 |bibcode=2000fist.conf...18F |doi=10.1007/10719504_3 |chapter=Death of Stellar Baryonic Dark Matter |series=ESO Astrophysics Symposia |last2=Fields |first2=Brian |last3=Graff |first3=David |year=2003 |title=The First Stars |isbn=978-3-540-67222-7 |citeseerx=10.1.1.256.6883|s2cid=119326375 }}</ref> * Detailed analysis of the small irregularities (anisotropies) in the [[cosmic microwave background]].<ref>{{cite journal |first1=L. |last1=Canetti |first2=M. |last2=Drewes |first3=M. |last3=Shaposhnikov |title=Matter and Antimatter in the Universe |journal=New J. Phys. |year=2012 |volume=14 |issue=9 |page=095012 |doi=10.1088/1367-2630/14/9/095012 |arxiv=1204.4186 |bibcode=2012NJPh...14i5012C|s2cid=119233888 }}</ref> Observations by [[WMAP]] and [[Planck (spacecraft)|Planck]] indicate that around five-sixths of the total matter is in a form that interacts significantly with ordinary matter or [[photon]]s only through gravitational effects. ==== Non-baryonic matter ==== Candidates for non-baryonic dark matter are hypothetical particles such as [[axion]]s, [[sterile neutrino]]s, [[weakly interacting massive particles]] (WIMPs), [[supersymmetric]] particles, or [[Geon (physics)|geons]].<ref>{{cite journal |last1=Guiot |first1=B |last2=Borquez |first2=A. |last3=Deur |first3=A. |last4=Werner |first4=K. |title=Graviballs and Dark Matter |journal=[[JHEP]] |year=2020 |volume=2020 |issue=11 |pages=159 |doi=10.1007/JHEP11(2020)159 |arxiv=2006.02534 |bibcode=2020JHEP...11..159G|s2cid=219303406 }}</ref><ref>{{cite journal |last1=Overduin |first1=J. M. |last2=Wesson |first2=P. S. |title=Dark Matter and Background Light |journal=Physics Reports |date=November 2004 |volume=402 |issue=5–6 |pages=267–406 |doi=10.1016/j.physrep.2004.07.006 |arxiv=astro-ph/0407207 |bibcode=2004PhR...402..267O |s2cid=1634052 }}</ref> The three neutrino types already observed are indeed abundant, and dark, and matter, but because their individual masses – however uncertain they may be – are almost certainly too tiny, they can only supply a small fraction of dark matter, due to limits derived from [[Observable universe|large-scale structure]] and high-[[redshift]] galaxies.<ref name="bertone merritt" /> Unlike baryonic matter, nonbaryonic matter did not contribute to the formation of the [[Chemical element|elements]] in the early universe ([[Big Bang nucleosynthesis]])<ref name="Copi 1995" /> and so its presence is revealed only via its gravitational effects, or [[weak lensing]]. In addition, if the particles of which it is composed are supersymmetric, they can undergo [[annihilation]] interactions with themselves, possibly resulting in observable by-products such as [[gamma rays]] and neutrinos (indirect detection).<ref name="bertone merritt">{{Cite journal |last1=Bertone |first1=G. |last2=Merritt |first2=D. |title=Dark Matter Dynamics and Indirect Detection |year=2005 |journal=[[Modern Physics Letters A]] |volume=20 |issue=14 |pages=1021–1036 |arxiv=astro-ph/0504422 |bibcode=2005MPLA...20.1021B |doi=10.1142/S0217732305017391|s2cid=119405319 }}</ref> ====Dark matter aggregation and dense dark matter objects==== If dark matter is composed of weakly-interacting particles, then an obvious question is whether it can form objects equivalent to [[planet]]s, [[star]]s, or [[black hole]]s. Historically, the answer has been it cannot,<ref name="cornell_ask">{{cite web |url=http://curious.astro.cornell.edu/about-us/95-the-universe/galaxies/general-questions/508-are-there-any-dark-stars-or-dark-galaxies-made-of-dark-matter-advanced |title=Are there any dark stars or dark galaxies made of dark matter? |website=Cornell University - Ask an Astronomer |archive-url=https://web.archive.org/web/20150302105015/http://curious.astro.cornell.edu/about-us/95-the-universe/galaxies/general-questions/508-are-there-any-dark-stars-or-dark-galaxies-made-of-dark-matter-advanced |archive-date=2 March 2015}}</ref><ref name="curio">{{cite journal |url=https://physics.aps.org/articles/v11/s15 |archive-url=https://archive.today/20201026224145/https://physics.aps.org/articles/v11/s15 |archive-date=26 October 2020 |date=1 February 2018 |title=Synopsis: A Way to Cool Dark Matter |journal=Physical Review Letters |volume=120 |issue=5 |pages=051102 |quote=One widely held belief about dark matter is it cannot cool off by radiating energy. If it could, then it might bunch together and create compact objects in the same way baryonic matter forms planets, stars, and galaxies. Observations so far suggest dark matter doesn't do that – it resides only in diffuse halos ... As a result, it is extremely unlikely there are very dense objects like stars made out of entirely (or even mostly) dark matter. |doi=10.1103/PhysRevLett.120.051102 |pmid=29481169 |last1=Buckley |first1=Matthew R. |last2=Difranzo |first2=Anthony |arxiv=1707.03829 |bibcode=2018PhRvL.120e1102B |s2cid=3757868 }}</ref><ref name="siegel">{{cite magazine |author-link=Ethan Siegel |author=Siegel, Ethan |date=28 October 2016 |url=https://www.forbes.com/sites/startswithabang/2016/10/28/why-doesnt-dark-matter-form-black-holes/#4e5014943de1 |title=Why doesn't dark matter form black holes? |magazine=Forbes }}</ref> because of two factors: ;It lacks an efficient means to lose energy<ref name="curio" /> :Ordinary matter forms dense objects because it has numerous ways to lose energy. Losing energy would be essential for object formation, because a particle that gains energy during compaction or falling "inward" under gravity, and cannot lose it any other way, will heat up and increase [[velocity]] and [[momentum]]. Dark matter appears to lack a means to lose energy, simply because it is not capable of interacting strongly in other ways except through gravity. The [[virial theorem]] suggests that such a particle would not stay bound to the gradually forming object – as the object began to form and compact, the dark matter particles within it would speed up and tend to escape. ;It lacks a range of interactions needed to form structures<ref name="siegel" /> :Ordinary matter interacts in many different ways, which allows the matter to form more complex structures. For example, stars form through gravity, but the particles within them interact and can emit energy in the form of [[neutrino]]s and [[electromagnetic radiation]] through [[nuclear fusion|fusion]] when they become energetic enough. [[Proton]]s and [[neutron]]s can bind via the [[strong interaction]] and then form [[atom]]s with [[electron]]s largely through [[electromagnetic interaction]]. There is no evidence that dark matter is capable of such a wide variety of interactions, since it seems to only interact through gravity (and possibly through some means no stronger than the [[weak interaction]], although until dark matter is better understood, this is only speculation). In 2015–2017, the idea that dense dark matter was composed of [[primordial black hole]]s made a comeback<ref>{{cite journal |last1=Cho |first1=Adrian |title=Is dark matter made of black holes? |journal=Science |date=9 February 2017 |doi=10.1126/science.aal0721}}</ref> following results of [[gravitational wave]] measurements which detected the merger of intermediate-mass black holes. Black holes with about 30&nbsp;solar masses are not predicted to form by either stellar collapse (typically less than 15&nbsp;solar masses) or by the merger of black holes in galactic centers (millions or billions of solar masses). It was proposed that the intermediate-mass black holes causing the detected merger formed in the hot dense early phase of the universe due to denser regions collapsing. A later survey of about a thousand supernovae detected no gravitational lensing events, when about eight would be expected if intermediate-mass primordial black holes above a certain mass range accounted for the majority of dark matter.<ref>{{cite web |url=http://astronomy.com/news/2018/10/can-black-holes-explain-dark-matter |title=Black holes can't explain dark matter |publisher=astronomy.com |date=18 October 2018 |access-date=7 January 2019}}</ref> The possibility that atom-sized primordial black holes account for a significant fraction of dark matter was ruled out by measurements of positron and electron fluxes outside the Sun's heliosphere by the Voyager&nbsp;1 spacecraft. Tiny black holes are theorized to emit [[Hawking radiation]]. However the detected fluxes were too low and did not have the expected energy spectrum, suggesting that tiny primordial black holes are not widespread enough to account for dark matter.<ref>{{cite web |url=https://www.science.org/content/article/aging-voyager-1-spacecraft-undermines-idea-dark-matter-tiny-black-holes |publisher=sciencemag.org |date=9 January 2019 |access-date=10 January 2019 |title=Aging Voyager&nbsp;1 spacecraft undermines idea that dark matter is tiny black holes}}</ref> Nonetheless, research and theories proposing dense dark matter accounts for dark matter continue as of 2018, including approaches to dark matter cooling,<ref>{{Cite web|url=https://www.newscientist.com/article/2160305-there-could-be-entire-stars-and-planets-made-out-of-dark-matter/|title=There could be entire stars and planets made out of dark matter|first=Shannon|last=Hall|website=New Scientist}}</ref><ref>{{cite journal |doi=10.1103/PhysRevLett.120.051102 |pmid=29481169 |title=Collapsed Dark Matter Structures |journal=Physical Review Letters |volume=120 |issue=5 |page=051102 |year=2018 |last1=Buckley |first1=Matthew R. |last2=Difranzo |first2=Anthony |arxiv=1707.03829 |bibcode=2018PhRvL.120e1102B|s2cid=3757868 }}</ref> and the question remains unsettled. In 2019, the lack of microlensing effects in the observation of Andromeda suggests that tiny black holes do not exist.<ref>{{cite journal |last1=Niikura |first1=Hiroko |title=Microlensing constraints on primordial black holes with Subaru/HSC Andromeda observations |journal=Nature Astronomy |volume=3 |issue=6 |pages=524–534 |date=1 April 2019 |doi=10.1038/s41550-019-0723-1 |bibcode=2019NatAs...3..524N|arxiv=1701.02151 |s2cid=118986293 }}</ref> However, there still exists a largely unconstrained mass range smaller than that which can be limited by optical microlensing observations, where primordial black holes may account for all dark matter.<ref>{{Cite journal|last1=Katz|first1=Andrey|last2=Kopp|first2=Joachim|last3=Sibiryakov|first3=Sergey|last4=Xue|first4=Wei|date=2018-12-05|title=Femtolensing by dark matter revisited|url=http://stacks.iop.org/1475-7516/2018/i=12/a=005?key=crossref.dcb1e788286d8d75d0c7e7f35fe588eb|journal=Journal of Cosmology and Astroparticle Physics|volume=2018|issue=12|pages=005|doi=10.1088/1475-7516/2018/12/005|issn=1475-7516|bibcode=2018JCAP...12..005K|arxiv=1807.11495|s2cid=119215426}}</ref><ref>{{Cite journal|last1=Montero-Camacho|first1=Paulo|last2=Fang|first2=Xiao|last3=Vasquez|first3=Gabriel|last4=Silva|first4=Makana|last5=Hirata|first5=Christopher M.|date=2019-08-23|title=Revisiting constraints on asteroid-mass primordial black holes as dark matter candidates|journal=Journal of Cosmology and Astroparticle Physics|volume=2019|issue=8|pages=031|doi=10.1088/1475-7516/2019/08/031|issn=1475-7516|bibcode=2019JCAP...08..031M|arxiv=1906.05950|s2cid=189897766}}</ref> === Free streaming length === Dark matter can be divided into ''cold'', ''warm'', and ''hot'' categories.<ref>{{cite book |first=Joseph |last=Silk |title=The Big Bang: Third Edition |chapter-url={{google books |plainurl=y |id=XLwe1lUmz5kC |page=82}} |date=2000 |publisher=Henry Holt and Company |isbn=978-0-8050-7256-3 |chapter=IX}}</ref> These categories refer to velocity rather than an actual temperature, indicating how far corresponding objects moved due to random motions in the early universe, before they slowed due to cosmic expansion – this is an important distance called the ''[[free streaming]] length'' (FSL). Primordial density fluctuations smaller than this length get washed out as particles spread from overdense to underdense regions, while larger fluctuations are unaffected; therefore this length sets a minimum scale for later structure formation. The categories are set with respect to the size of a [[protogalaxy]] (an object that later evolves into a [[dwarf galaxy]]): Dark matter particles are classified as cold, warm, or hot according to their FSL; much smaller (cold), similar to (warm), or much larger (hot) than a protogalaxy.<ref>{{cite journal |first1=N. |last1=Vittorio |author2=J. Silk |date=1984 |journal=Astrophysical Journal Letters |volume=285 |pages=L39–L43 |title=Fine-scale anisotropy of the cosmic microwave background in a universe dominated by cold dark matter |doi=10.1086/184361 |bibcode=1984ApJ...285L..39V}}</ref><ref>{{Cite journal |first1=Masayuki |last1=Umemura |author2=Satoru Ikeuchi |title=Formation of Subgalactic Objects within Two-Component Dark Matter |date=1985 |journal=Astrophysical Journal |volume=299 |pages=583–592 |doi=10.1086/163726 |bibcode=1985ApJ...299..583U}}</ref> Mixtures of the above are also possible: a theory of [[mixed dark matter]] was popular in the mid-1990s, but was rejected following the discovery of [[dark energy]].{{Citation needed|date=April 2016}} Cold dark matter leads to a bottom-up formation of structure with galaxies forming first and galaxy clusters at a latter stage, while hot dark matter would result in a top-down formation scenario with large matter aggregations forming early, later fragmenting into separate galaxies;{{Clarify|date=September 2018}} the latter is excluded by high-redshift galaxy observations.<ref name="bertone hooper silk" /> ==== Fluctuation spectrum effects ==== These categories also correspond to [[fluctuation spectrum]] effects {{explain|reason=say what's fluctuating, and whether with time or distance|date=September 2021}} and the interval following the Big Bang at which each type became non-relativistic. Davis ''et al.'' wrote in 1985:<ref name=Davis-Efstath-etal-1985>{{cite journal |last1=Davis |first1=M. |author2=Efstathiou, G. |author3=Frenk, C.S. |author4=White, S.D.M. |date=15 May 1985 |title=The evolution of large-scale structure in a universe dominated by cold dark matter |journal=Astrophysical Journal |volume=292 |pages=371–394 |doi=10.1086/163168 |bibcode=1985ApJ...292..371D}}</ref> {{quote| Candidate particles can be grouped into three categories on the basis of their effect on the [[Mixed dark matter|fluctuation spectrum]] (Bond ''et al.'' 1983). If the dark matter is composed of abundant light particles which remain relativistic until shortly before recombination, then it may be termed "hot". The best candidate for hot dark matter is a neutrino ... A second possibility is for the dark matter particles to interact more weakly than neutrinos, to be less abundant, and to have a mass of order 1&nbsp;keV. Such particles are termed "warm dark matter", because they have lower thermal velocities than massive neutrinos ... there are at present few candidate particles which fit this description. [[Gravitino]]s and [[photino]]s have been suggested (Pagels and Primack 1982; Bond, Szalay and Turner 1982) ... Any particles which became nonrelativistic very early, and so were able to diffuse a negligible distance, are termed "cold" dark matter (CDM). There are many candidates for CDM including supersymmetric particles.<br/> {{right| — Davis, Efstathiou, Frenk, & White (1985)<ref name=Davis-Efstath-etal-1985/>}} }} ==== Alternative definitions ==== Another approximate dividing line is warm dark matter became non-relativistic when the universe was approximately 1&nbsp;year old and 1&nbsp;millionth of its present size and in the [[radiation-dominated era]] (photons and neutrinos), with a photon temperature 2.7&nbsp;million&nbsp;Kelvins. Standard physical cosmology gives the [[particle horizon]] size as 2&nbsp;''c&nbsp;t'' (speed of light multiplied by time) in the radiation-dominated era, thus 2&nbsp;light-years. A region of this size would expand to 2&nbsp;million light-years today (absent structure formation). The actual FSL is approximately 5&nbsp;times the above length, since it continues to grow slowly as particle velocities decrease inversely with the scale factor after they become non-relativistic. In this example the FSL would correspond to 10&nbsp;million light-years, or 3&nbsp;mega[[parsec]]s, today, around the size containing an average large galaxy. The 2.7&nbsp;million&nbsp;[[Kelvin (unit)|K]] photon temperature gives a typical photon energy of 250&nbsp;electronvolts, thereby setting a typical mass scale for warm dark matter: particles much more massive than this, such as GeV–TeV mass [[weakly interacting massive particles|WIMPs]], would become non-relativistic much earlier than one year after the Big Bang and thus have FSLs much smaller than a protogalaxy, making them cold. Conversely, much lighter particles, such as neutrinos with masses of only a few eV, have FSLs much larger than a protogalaxy, thus qualifying them as hot. ==== Cold dark matter ==== {{Main|Cold dark matter}} [[Cold dark matter]] offers the simplest explanation for most cosmological observations. It is dark matter composed of constituents with an FSL much smaller than a protogalaxy. This is the focus for dark matter research, as hot dark matter does not seem capable of supporting galaxy or galaxy cluster formation, and most particle candidates slowed early. The constituents of cold dark matter are unknown. Possibilities range from large objects like MACHOs (such as black holes<ref name=Hawkins>{{Cite journal |last1=Hawkins |first1=M.R.S. |title=The case for primordial black holes as dark matter |doi=10.1111/j.1365-2966.2011.18890.x |journal=Monthly Notices of the Royal Astronomical Society |volume=415 |issue=3 |pages=2744–2757 |year=2011 |arxiv=1106.3875 |bibcode=2011MNRAS.415.2744H|s2cid=119261917 }}</ref> and [[Preon star]]s<ref name="Phys.Lett.B616,1(2005">{{cite journal |last1=Hansson |first1=J. |last2=Sandin |first2=F. |year=2005 |title=Preon stars: a new class of cosmic compact objects |journal=[[Physics Letters B]] |volume=616 |issue=1–2 |pages=1–7 |arxiv=astro-ph/0410417 |doi=10.1016/j.physletb.2005.04.034 |bibcode=2005PhLB..616....1H |s2cid=119063004 }}</ref>) or [[Robust associations of massive baryonic objects|RAMBOs]] (such as clusters of brown dwarfs), to new particles such as [[weakly interacting massive particles|WIMPs]] and [[axion]]s. Studies of [[Big Bang nucleosynthesis]] and gravitational lensing convinced most cosmologists<ref name="bertone hooper silk" /><ref name=Carr/><ref name=Peter/><ref name=Garrett>{{cite journal |doi=10.1155/2011/968283 |quote=MACHOs can only account for a very small percentage of the nonluminous mass in our galaxy, revealing that most dark matter cannot be strongly concentrated or exist in the form of baryonic astrophysical objects. Although microlensing surveys rule out baryonic objects like brown dwarfs, black holes, and neutron stars in our galactic halo, can other forms of baryonic matter make up the bulk of dark matter? The answer, surprisingly, is 'no'&nbsp;... |title=Dark Matter: A Primer |year=2011 |last1=Garrett |first1=Katherine |last2=Dūda |first2=Gintaras |journal=Advances in Astronomy |volume=2011 |issue=968283 |pages=1–22 |arxiv=1006.2483 |bibcode=2011AdAst2011E...8G|s2cid=119180701 |doi-access=free }}</ref><ref name=Bertone>{{Cite journal |last1=Bertone |first1=G. |title=The moment of truth for WIMP dark matter |doi=10.1038/nature09509 |journal=Nature |volume=468 |issue=7322 |pages=389–393 |year=2010 |pmid=21085174 |arxiv=1011.3532 |bibcode=2010Natur.468..389B |s2cid=4415912 }}</ref><ref name=Olive>{{cite arXiv |eprint=astro-ph/0301505 |page=21 |last1=Olive |first1=Keith A |title=TASI Lectures on Dark Matter |year=2003}}</ref> that MACHOs<ref name="Carr">{{cite journal |last1=Carr |first1=B.J. |display-authors=etal |year=2010 |title=New cosmological constraints on primordial black holes |journal=Physical Review D |volume=81 |page=104019 |doi=10.1103/PhysRevD.81.104019 |arxiv=0912.5297 |bibcode=2010PhRvD..81j4019C |issue=10|s2cid=118946242 }}</ref><ref name="Garrett" /> cannot make up more than a small fraction of dark matter.<ref name="Copi 1995" /><ref name=Carr /> According to A. Peter: "...&nbsp;the only ''really plausible'' dark-matter candidates are new particles."<ref name=Peter>{{cite arXiv |last=Peter |first=A.H.G. |title=Dark matter: A brief review |eprint=1201.3942 |class=astro-ph.CO |year=2012}}</ref> The 1997 [[DAMA/NaI]] experiment and its successor [[DAMA/LIBRA]] in 2013, claimed to directly detect dark matter particles passing through the Earth, but many researchers remain skeptical, as negative results from similar experiments seem incompatible with the DAMA results. Many [[supersymmetry|supersymmetric]] models offer dark matter candidates in the form of the WIMPy [[Lightest Supersymmetric Particle]] (LSP).<ref>{{Cite journal |title=Supersymmetric dark matter |journal=Physics Reports |date=1 March 1996 |pages=195–373 |volume=267 |issue=5–6 |doi=10.1016/0370-1573(95)00058-5 |first1=Gerard |last1=Jungman |first2=Marc |last2=Kamionkowski |first3=Kim |last3=Griest |arxiv=hep-ph/9506380 |bibcode=1996PhR...267..195J|s2cid=119067698 }}</ref> Separately, heavy sterile neutrinos exist in non-supersymmetric extensions to the [[Standard Model|standard model]] which explain the small [[neutrino]] mass through the [[seesaw mechanism]]. ==== Warm dark matter ==== {{Main|Warm dark matter}} [[Warm dark matter]] comprises particles with an FSL comparable to the size of a protogalaxy. Predictions based on warm dark matter are similar to those for cold dark matter on large scales, but with less small-scale density perturbations. This reduces the predicted abundance of dwarf galaxies and may lead to lower density of dark matter in the central parts of large galaxies. Some researchers consider this a better fit to observations. A challenge for this model is the lack of particle candidates with the required mass ≈&nbsp;300&nbsp;eV to 3000&nbsp;eV.{{Citation needed|date=April 2016}} No known particles can be categorized as warm dark matter. A postulated candidate is the [[sterile neutrino]]: A heavier, slower form of neutrino that does not interact through the [[Weak interaction|weak force]], unlike other neutrinos. Some modified gravity theories, such as [[scalar–tensor–vector gravity]], require "warm" dark matter to make their equations work. ==== Hot dark matter ==== {{Main|Hot dark matter}} [[Hot dark matter]] consists of particles whose FSL is much larger than the size of a protogalaxy. The [[neutrino]] qualifies as such particle. They were discovered independently, long before the hunt for dark matter: they were postulated in 1930, and [[Cowan–Reines neutrino experiment|detected in 1956]]. Neutrinos' [[Neutrino mass|mass]] is less than 10{{sup|−6}} that of an [[electron]]. Neutrinos interact with normal matter only via gravity and the [[Weak interaction|weak force]], making them difficult to detect (the weak force only works over a small distance, thus a neutrino triggers a weak force event only if it hits a nucleus head-on). This makes them “[[WISP (particle physics)|weakly interacting slender particles]]” ([[WISP (particle physics)|WISP]]s), as opposed to WIMPs. The three known [[Flavour (particle physics)|flavours]] of neutrinos are the ''electron'', ''muon'', and ''tau''. Their masses are slightly different. Neutrinos oscillate among the flavours as they move. It is hard to determine an exact [[Upper and lower bounds|upper bound]] on the collective average mass of the three neutrinos (or for any of the three individually). For example, if the average neutrino mass were over 50&nbsp;[[Electronvolt|eV]]/c{{sup|2}} (less than 10{{sup|−5}} of the mass of an electron), the universe would collapse. <ref>{{Cite journal |last1=Duan |first1=Huaiyu |last2=Fuller |first2=George M. |last3=Qian |first3=Yong-Zhong |date=2010-11-23 |title=Collective Neutrino Oscillations |url=https://www.annualreviews.org/doi/10.1146/annurev.nucl.012809.104524 |journal=Annual Review of Nuclear and Particle Science |language=en |volume=60 |issue=1 |pages=569–594 |doi=10.1146/annurev.nucl.012809.104524 |arxiv=1001.2799 |bibcode=2010ARNPS..60..569D |s2cid=118656162 |issn=0163-8998}}</ref> CMB data and other methods indicate that their average mass probably does not exceed 0.3&nbsp;eV/c{{sup|2}}. Thus, observed neutrinos cannot explain dark matter.<ref> {{cite web |title=Neutrinos as dark matter |date=21 September 1998 |publisher=Astro.ucla.edu |url=http://www.astro.ucla.edu/~wright/neutrinos.html |access-date=6 January 2011 }} </ref> Because galaxy-size density fluctuations get washed out by free-streaming, hot dark matter implies the first objects that can form are huge [[supercluster]]-size pancakes, which then fragment into galaxies. [[List of deep fields|Deep-field observations]] show instead that galaxies formed first, followed by clusters and superclusters as galaxies clump together. == Detection of dark matter particles == If dark matter is made up of subatomic particles, then millions, possibly billions, of such particles must pass through every square centimeter of the Earth each second.<ref name="Gaitskell 2004">{{cite journal |last=Gaitskell |first=Richard J. |s2cid=11316578 |title=Direct Detection of Dark Matter |journal=[[Annual Review of Nuclear and Particle Science]] |volume=54 |pages=315–359 |bibcode=2004ARNPS..54..315G |date=2004 |doi=10.1146/annurev.nucl.54.070103.181244|doi-access=free}}</ref><ref name="Number per second">{{cite web |title=Neutralino Dark Matter |url=http://www.picassoexperiment.ca/dm_neutralino.php |access-date=26 December 2011}} {{cite web |title=WIMPs and MACHOs |url=http://www.astro.caltech.edu/~george/ay20/eaa-wimps-machos.pdf |archive-url=https://web.archive.org/web/20060923123531/http://www.astro.caltech.edu/~george/ay20/eaa-wimps-machos.pdf |archive-date=2006-09-23 |url-status=live |last=Griest |first=Kim |access-date=26 December 2011}}</ref> Many experiments aim to test this hypothesis. Although [[Weakly interacting massive particles|WIMPs]] have been the main search candidates,<ref name="bertone hooper silk" /> [[axion]]s have drawn renewed attention, with the [[Axion Dark Matter Experiment]] (ADMX) searches for axions and many more planned in the future.<ref name="Chadha-Day et al">{{cite journal|title=Axion dark matter: What is it and why now?|author1=Francesca Chadha-Day|author2=John Ellis|author3=David J. E. Marsh|journal=Science Advances|volume=8|issue=8|doi=10.1126/sciadv.abj3618|date=23 February 2022| pages=eabj3618 | pmid=35196098 | pmc=8865781 | arxiv=2105.01406 | bibcode=2022SciA....8J3618C }}</ref> Another candidate is heavy [[hidden sector]] particles which only interact with ordinary matter via gravity. These experiments can be divided into two classes: direct detection experiments, which search for the scattering of dark matter particles off atomic nuclei within a detector; and indirect detection, which look for the products of dark matter particle annihilations or decays.<ref name="bertone merritt" /> === Direct detection === {{details|Weakly interacting massive particles#Direct detection}} {{main|Direct detection of dark matter}} Direct detection experiments aim to observe low-energy recoils (typically a few [[keV]]s) of nuclei induced by interactions with particles of dark matter, which (in theory) are passing through the Earth. After such a recoil the nucleus will emit energy in the form of [[scintillation (physics)|scintillation]] light or [[phonon]]s, as they pass through sensitive detection apparatus. To do so effectively, it is crucial to maintain an extremely low background, which is the reason why such experiments typically operate deep underground, where interference from [[cosmic ray]]s is minimized. Examples of underground laboratories with direct detection experiments include the [[Stawell Underground Physics Laboratory|Stawell mine]], the [[Soudan mine]], the [[SNOLAB]] underground laboratory at [[Greater Sudbury|Sudbury]], the [[Gran Sasso National Laboratory]], the [[Canfranc Underground Laboratory]], the [[Boulby Underground Laboratory]], the [[Deep Underground Science and Engineering Laboratory]] and the [[China Jinping Underground Laboratory]]. These experiments mostly use either cryogenic or noble liquid detector technologies. Cryogenic detectors operating at temperatures below 100&nbsp;mK, detect the heat produced when a particle hits an atom in a crystal absorber such as [[germanium]]. [[Noble gas|Noble liquid]] detectors detect [[scintillation (physics)|scintillation]] produced by a particle collision in liquid [[xenon]] or [[argon]]. Cryogenic detector experiments include: [[Cryogenic Dark Matter Search|CDMS]], [[Cryogenic Rare Event Search with Superconducting Thermometers|CRESST]], [[EDELWEISS]], [[European Underground Rare Event Calorimeter Array|EURECA]]. Noble liquid experiments include [[LZ experiment|LZ]], [[XENON]], [[DEAP]], [[ArDM]], [[WIMP Argon Programme|WARP]], [[DarkSide (dark matter experiment)|DarkSide]], [[PandaX]], and LUX, the [[Large Underground Xenon experiment]]. Both of these techniques focus strongly on their ability to distinguish background particles (which predominantly scatter off electrons) from dark matter particles (that scatter off nuclei). Other experiments include [[SIMPLE (dark matter)|SIMPLE]] and [[PICASSO (dark matter)|PICASSO]]. Currently there has been no well-established claim of dark matter detection from a direct detection experiment, leading instead to strong upper limits on the mass and interaction cross section with nucleons of such dark matter particles.<ref>{{cite journal |last1=Drees |first1=M. |last2=Gerbier |first2=G. |title=Dark Matter |journal=Chin. Phys. C |date=2015 |volume=38 |page=090001 |url=http://pdg.lbl.gov/2015/reviews/rpp2015-rev-dark-matter.pdf |archive-url=https://web.archive.org/web/20160722093442/http://pdg.lbl.gov/2015/reviews/rpp2015-rev-dark-matter.pdf |archive-date=2016-07-22 |url-status=live}}</ref> The [[DAMA/NaI]] and more recent [[DAMA/LIBRA]] experimental collaborations have detected an annual modulation in the rate of events in their detectors,<ref>{{Cite journal |last1=Bernabei |first1=R. |title=First results from DAMA/LIBRA and the combined results with DAMA/NaI |journal=Eur. Phys. J. C |volume=56 |issue=3 |pages=333–355 |year=2008 |doi=10.1140/epjc/s10052-008-0662-y |arxiv=0804.2741 |last2=Belli |first2=P. |last3=Cappella |first3=F. |last4=Cerulli |first4=R. |last5=Dai |first5=C.J. |last6=d’Angelo |first6=A. |last7=He |first7=H.L. |last8=Incicchitti |first8=A. |last9=Kuang |first9=H.H. |last10=Ma |first10=J.M. |last11=Montecchia |first11=F. |last12=Nozzoli |first12=F. |last13=Prosperi |first13=D. |last14=Sheng |first14=X.D. |last15=Ye |display-authors=6 |first15=Z.P. |bibcode=2008EPJC...56..333B|s2cid=14354488 }}</ref><ref>{{Cite journal |doi=10.1103/PhysRevD.33.3495 |pmid=9956575 |author1=Drukier, A. |author2=Freese, K. |author3=Spergel, D. |title=Detecting Cold Dark Matter Candidates |journal=Physical Review D |volume=33 |issue=12 |pages=3495–3508 |date=1986 |bibcode=1986PhRvD..33.3495D}}</ref> which they claim is due to dark matter. This results from the expectation that as the Earth orbits the Sun, the velocity of the detector relative to the [[dark matter halo]] will vary by a small amount. This claim is so far unconfirmed and in contradiction with negative results from other experiments such as LUX, SuperCDMS<ref>{{cite journal |last1=Davis |first1=Jonathan H. |title=The past and future of light dark matter direct detection |journal=Int. J. Mod. Phys. A |year=2015 |volume=30 |issue=15 |page=1530038 |doi=10.1142/S0217751X15300380 |arxiv=1506.03924 |bibcode=2015IJMPA..3030038D |s2cid=119269304 }}</ref> and XENON100.<ref>{{cite journal |last1=Aprile |first1=E. |title=Search for electronic recoil event rate modulation with 4&nbsp;years of XENON100 data |journal=Phys. Rev. Lett. |year=2017 |volume=118 |issue=10 |page=101101 |doi=10.1103/PhysRevLett.118.101101 |pmid=28339273 |arxiv=1701.00769|bibcode=2017PhRvL.118j1101A |s2cid=206287497 }}</ref> A special case of direct detection experiments covers those with directional sensitivity. This is a search strategy based on the motion of the Solar System around the [[Galactic Center]].<ref name="apssyn">{{cite news |last=Stonebraker |first=Alan |title=Synopsis: Dark-Matter Wind Sways through the Seasons |newspaper=Physics – Synopses |publisher=[[American Physical Society]] |date=3 January 2014 |doi=10.1103/PhysRevLett.112.011301 }}</ref><ref name="samlee">{{cite journal |last1=Lee |first1=Samuel K. |first2=Mariangela |last2=Lisanti |first3=Annika H.G.|last3=Peter |first4=Benjamin R.|last4=Safdi |doi=10.1103/PhysRevLett.112.011301 |arxiv=1308.1953 |title=Effect of Gravitational Focusing on Annual Modulation in Dark-Matter Direct-Detection Experiments |date=3 January 2014 |journal=Phys. Rev. Lett. |volume=112 |issue=1 |page=011301 [5 pages] |bibcode=2014PhRvL.112a1301L |pmid=24483881|s2cid=34109648 }}</ref><ref name="dmgsheff">{{cite news |last=The Dark Matter Group |title=An Introduction to Dark Matter |newspaper=Dark Matter Research |location=Sheffield |publisher=University of Sheffield|url=http://www.hep.shef.ac.uk/research/dm/intro.php |access-date=7 January 2014}}</ref><ref name="Kavli">{{cite news |quote=Scientists at Kavli MIT are working on ... a tool to track the movement of dark matter. |title=Blowing in the Wind |newspaper=Kavli News |location=Sheffield |publisher=[[Kavli Foundation (United States)|Kavli Foundation]] |url=http://www.kavlifoundation.org/science-spotlights/blowing-wind |access-date=7 January 2014}}</ref> A low-pressure [[time projection chamber]] makes it possible to access information on recoiling tracks and constrain WIMP-nucleus kinematics. WIMPs coming from the direction in which the Sun travels (approximately towards [[Cygnus (constellation)|Cygnus]]) may then be separated from background, which should be isotropic. Directional dark matter experiments include [[Dark Matter Time Projection Chamber|DMTPC]], [[Directional Recoil Identification From Tracks|DRIFT]], Newage and MIMAC. === Indirect detection === {{main|Indirect detection of dark matter}} [[File:Collage of six cluster collisions with dark matter maps.jpg|thumb|Collage of six cluster collisions with dark matter maps. The clusters were observed in a study of how dark matter in clusters of galaxies behaves when the clusters collide.<ref>{{cite web |title=Dark matter even darker than once thought |url=http://www.spacetelescope.org/news/heic1506/ |access-date=16 June 2015 |website=Space Telescope Science Institute}}</ref>]] [[File:Turning Black Holes into Dark Matter Labs.webm|thumb|Video about the potential [[Gamma-ray astronomy|gamma-ray detection]] of dark matter [[annihilation]] around [[supermassive black hole]]s. ''(Duration 0:03:13, also see file description.)'']] Indirect detection experiments search for the products of the self-annihilation or decay of dark matter particles in outer space. For example, in regions of high dark matter density (e.g., the [[Galactic Center|centre of our galaxy]]) two dark matter particles could [[Annihilation|annihilate]] to produce [[gamma ray]]s or Standard Model particle–antiparticle pairs.<ref name="Bertone2010">{{cite book |first=Gianfranco |last=Bertone |title=Particle Dark Matter: Observations, Models and Searches |chapter-url=https://books.google.com/books?id=JkUgAwAAQBAJ&pg=PA83 |year=2010 |publisher=Cambridge University Press |pages=83–104 |chapter=Dark Matter at the Centers of Galaxies |arxiv=1001.3706 |isbn=978-0-521-76368-4 |bibcode=2010arXiv1001.3706M}}</ref> Alternatively, if a dark matter particle is unstable, it could decay into Standard Model (or other) particles. These processes could be detected indirectly through an excess of gamma rays, [[antiproton]]s or [[positron]]s emanating from high density regions in our galaxy or others.<ref>{{cite journal |last1=Ellis |first1=J. |last2=Flores |first2=R.A. |last3=Freese |first3=K. |last4=Ritz |first4=S. |last5=Seckel |first5=D. |last6=Silk |first6=J. |doi=10.1016/0370-2693(88)91385-8 |title=Cosmic ray constraints on the annihilations of relic particles in the galactic halo |journal=Physics Letters B |volume=214 |issue=3 |pages=403–412 |year=1988 |bibcode=1988PhLB..214..403E |url=http://cds.cern.ch/record/190709/files/198809398.pdf |archive-url=https://web.archive.org/web/20180728133226/http://cds.cern.ch/record/190709/files/198809398.pdf |archive-date=2018-07-28 |url-status=live}}</ref> A major difficulty inherent in such searches is that various astrophysical sources can mimic the signal expected from dark matter, and so multiple signals are likely required for a conclusive discovery.<ref name="bertone hooper silk" /><ref name="bertone merritt" /> A few of the dark matter particles passing through the Sun or Earth may scatter off atoms and lose energy. Thus dark matter may accumulate at the center of these bodies, increasing the chance of collision/annihilation. This could produce a distinctive signal in the form of high-energy [[neutrino]]s.<ref>{{cite journal |doi=10.1016/0370-2693(86)90349-7 |author=Freese, K. |title=Can Scalar Neutrinos or Massive Dirac Neutrinos be the Missing Mass? |journal=Physics Letters B |volume=167 |issue=3 |pages=295–300 |date=1986 |bibcode=1986PhLB..167..295F}}</ref> Such a signal would be strong indirect proof of WIMP dark matter.<ref name="bertone hooper silk" /> High-energy neutrino telescopes such as [[Antarctic Muon And Neutrino Detector Array|AMANDA]], [[IceCube]] and [[ANTARES (telescope)|ANTARES]] are searching for this signal.{{sfn|Randall|2015|p=298}} The detection by [[LIGO]] in [[GW150914|September 2015]] of gravitational waves opens the possibility of observing dark matter in a new way, particularly if it is in the form of [[primordial black holes]].<ref>{{cite magazine |url=https://www.newscientist.com/article/2077800-what-will-gravitational-waves-tell-us-about-the-universe |title=Surfing gravity's waves |magazine=New Scientist |first=Joshua |last=Sokol |display-authors=etal |issue=3061 |date=20 February 2016}}</ref><ref>{{cite web |publisher=Johns Hopkins University |title=Did gravitational wave detector find dark matter? |date=15 June 2016 |url=http://releases.jhu.edu/2016/06/15/did-gravitational-wave-detector-find-dark-matter/ |access-date=20 June 2015 |quote=While their existence has not been established with certainty, primordial black holes have in the past been suggested as a possible solution to the dark matter mystery. Because there is so little evidence of them, though, the primordial black hole–dark matter hypothesis has not gained a large following among scientists. The LIGO findings, however, raise the prospect anew, especially as the objects detected in that experiment conform to the mass predicted for dark matter. Predictions made by scientists in the past held conditions at the birth of the universe would produce many of these primordial black holes distributed approximately evenly in the universe, clustering in halos around galaxies. All this would make them good candidates for dark matter.}}</ref><ref>{{cite journal |last1=Bird |first1=Simeon |last2=Cholis |first2=Illian |year=2016 |title=Did LIGO detect dark matter? |journal=Physical Review Letters |volume=116 |issue=20 |page=201301 |doi=10.1103/PhysRevLett.116.201301 |pmid=27258861 |bibcode=2016PhRvL.116t1301B |arxiv=1603.00464|s2cid=23710177 }}</ref> Many experimental searches have been undertaken to look for such emission from dark matter annihilation or decay, examples of which follow. The [[Energetic Gamma Ray Experiment Telescope]] observed more gamma rays in 2008 than expected from the [[Milky Way]], but scientists concluded this was most likely due to incorrect estimation of the telescope's sensitivity.<ref>{{Cite journal |title=The likely cause of the EGRET GeV anomaly and its implications |journal=Astroparticle Physics |volume=29 |issue=1 |year=2008 |pages=25–29 |doi=10.1016/j.astropartphys.2007.11.002 |arxiv=0705.4311 |first1=F.W. |last1=Stecker |last2=Hunter |first2=S. |last3=Kniffen |first3=D. |bibcode=2008APh....29...25S|s2cid=15107441 }}</ref> The [[Fermi Gamma-ray Space Telescope]] is searching for similar gamma rays.<ref>{{Cite journal |title=The large area telescope on the Fermi Gamma-ray Space Telescope Mission |journal=Astrophysical Journal |volume=697 |issue=2 |year=2009 |pages=1071–1102 |doi=10.1088/0004-637X/697/2/1071 |arxiv=0902.1089 |first1=W.B. |last1=Atwood |last2=Abdo |first2=A.A. |last3=Ackermann |first3=M. |last4=Althouse |first4=W. |last5=Anderson |first5=B. |last6=Axelsson |first6=M. |last7=Baldini |first7=L. |last8=Ballet |first8=J. |last9=Band |first9=D.L. |display-authors=6 |bibcode=2009ApJ...697.1071A|s2cid=26361978 }}</ref> In April 2012, an analysis of previously available data from its [[Fermi LAT|Large Area Telescope]] instrument produced statistical evidence of a 130&nbsp;GeV signal in the gamma radiation coming from the center of the Milky Way.<ref>{{cite journal |doi=10.1088/1475-7516/2012/08/007 |last=Weniger |first=Christoph |title=A tentative gamma-ray line from dark matter annihilation at the Fermi Large Area Telescope |journal=Journal of Cosmology and Astroparticle Physics |issue=8 |date=2012 |arxiv=1204.2797 |volume=2012 |page=7 |bibcode=2012JCAP...08..007W|s2cid=119229841 }}</ref> WIMP annihilation was seen as the most probable explanation.<ref>{{cite web |url=http://physicsworld.com/cws/article/news/2012/apr/24/gamma-rays-hint-at-dark-matter |title=Gamma rays hint at dark matter |last1=Cartlidge |first1=Edwin |date=24 April 2012 |publisher=Institute of Physics |access-date=23 April 2013}}</ref> At higher energies, [[IACT|ground-based gamma-ray telescopes]] have set limits on the annihilation of dark matter in [[dwarf spheroidal galaxies]]<ref>{{Cite journal |last1=Albert |first1=J. |last2=Aliu |first2=E. |last3=Anderhub |first3=H. |last4=Antoranz |first4=P. |last5=Backes |first5=M. |last6=Baixeras |first6=C. |last7=Barrio |first7=J.A. |last8=Bartko |first8=H. |last9=Bastieri |first9=D. |last10=Becker |first10=J.K. |last11=Bednarek |first11=W. |last12=Berger |first12=K. |last13=Bigongiari |first13=C. |last14=Biland |first14=A. |last15=Bock |first15=R.K. |last16=Bordas |first16=P. |last17=Bosch‐Ramon |first17=V. |last18=Bretz |first18=T. |last19=Britvitch |first19=I. |last20=Camara |first20=M. |last21=Carmona |first21=E. |last22=Chilingarian |first22=A. |last23=Commichau |first23=S. |last24=Contreras |first24=J.L. |last25=Cortina |first25=J. |last26=Costado |first26=M.T. |last27=Curtef |first27=V. |last28=Danielyan |first28=V. |last29=Dazzi |first29=F. |last30=De Angelis |first30=A. |display-authors=6 |title=Upper Limit for γ‐Ray Emission above 140&nbsp;GeV from the Dwarf Spheroidal Galaxy Draco |doi=10.1086/529135 |journal=The Astrophysical Journal |volume=679 |issue=1 |pages=428–431 |year=2008 |arxiv=0711.2574 |bibcode=2008ApJ...679..428A|s2cid=15324383 }}</ref> and in clusters of galaxies.<ref>{{Cite journal |last1=Aleksić |first1=J. |last2=Antonelli |first2=L.A. |last3=Antoranz |first3=P. |last4=Backes |first4=M. |last5=Baixeras |first5=C. |last6=Balestra |first6=S. |last7=Barrio |first7=J.A. |last8=Bastieri |first8=D. |last9=González |first9=J.B. |last10=Bednarek |first10=W. |last11=Berdyugin |first11=A. |last12=Berger |first12=K. |last13=Bernardini |first13=E. |last14=Biland |first14=A. |last15=Bock |first15=R.K. |last16=Bonnoli |first16=G. |last17=Bordas |first17=P. |last18=Tridon |first18=D.B. |last19=Bosch-Ramon |first19=V. |last20=Bose |first20=D. |last21=Braun |first21=I. |last22=Bretz |first22=T. |last23=Britzger |first23=D. |last24=Camara |first24=M. |last25=Carmona |first25=E. |last26=Carosi |first26=A. |last27=Colin |first27=P. |last28=Commichau |first28=S. |last29=Contreras |first29=J.L. |last30=Cortina |first30=J. |display-authors=6 |title=Magic Gamma-Ray Telescope observation of the Perseus Cluster of galaxies: Implications for cosmic rays, dark matter, and NGC&nbsp;1275 |doi=10.1088/0004-637X/710/1/634 |journal=The Astrophysical Journal |volume=710 |issue=1 |pages=634–647 |year=2010 |arxiv=0909.3267 |bibcode=2010ApJ...710..634A|s2cid=53120203 }}</ref> The [[Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics|PAMELA]] experiment (launched in 2006) detected excess [[positron]]s. They could be from dark matter annihilation or from [[pulsar]]s. No excess [[antiproton]]s were observed.<ref>{{cite journal |last1=Adriani |first1=O. |last2=Barbarino |first2=G.C. |last3=Bazilevskaya |first3=G.A. |last4=Bellotti |first4=R. |last5=Boezio |first5=M. |last6=Bogomolov |first6=E.A. |last7=Bonechi |first7=L. |last8=Bongi |first8=M. |last9=Bonvicini |first9=V. |last10=Bottai |doi=10.1038/nature07942 |first10=S. |last11=Bruno |first11=A. |last12=Cafagna |first12=F. |last13=Campana |first13=D. |last14=Carlson |first14=P. |last15=Casolino |first15=M. |last16=Castellini |first16=G. |last17=De Pascale |first17=M.P. |last18=De Rosa |first18=G. |last19=De Simone |first19=N. |last20=Di Felice |first20=V. |last21=Galper |first21=A.M. |last22=Grishantseva |first22=L. |last23=Hofverberg |first23=P. |last24=Koldashov |first24=S.V. |last25=Krutkov |first25=S.Y. |last26=Kvashnin |first26=A.N. |last27=Leonov |first27=A. |last28=Malvezzi |first28=V. |last29=Marcelli |first29=L. |last30=Menn |first30=W. |display-authors=6 |title=An anomalous positron abundance in cosmic rays with energies 1.5–100&nbsp;GeV |journal=Nature |volume=458 |issue=7238 |pages=607–609 |year=2009 |pmid=19340076 |arxiv=0810.4995 |bibcode=2009Natur.458..607A|s2cid=11675154 }}</ref> In 2013 results from the [[Alpha Magnetic Spectrometer]] on the [[International Space Station]] indicated excess high-energy [[cosmic ray]]s which could be due to dark matter annihilation.<ref name="APS-20130403">{{cite journal |title=First Result from the Alpha Magnetic Spectrometer on the International Space Station: Precision Measurement of the Positron Fraction in Primary Cosmic Rays of 0.5–350&nbsp;GeV |date=3 April 2013 |journal=[[Physical Review Letters]] |author=Aguilar, M. |collaboration=AMS Collaboration |doi=10.1103/PhysRevLett.110.141102 |bibcode=2013PhRvL.110n1102A |display-authors=etal |volume=110 |issue=14 |pmid=25166975 |page=141102|doi-access=free }}</ref><ref name="AMS-20130403">{{cite web |title=First Result from the Alpha Magnetic Spectrometer Experiment |url=http://www.ams02.org/2013/04/first-results-from-the-alpha-magnetic-spectrometer-ams-experiment/ |date=3 April 2013 |author=AMS Collaboration |access-date=3 April 2013 |archive-url=https://web.archive.org/web/20130408185229/http://www.ams02.org/2013/04/first-results-from-the-alpha-magnetic-spectrometer-ams-experiment/ |archive-date=8 April 2013 |url-status=dead }}</ref><ref name="AP-20130403">{{cite news |last1=Heilprin |first1=John |last2=Borenstein |first2=Seth |title=Scientists find hint of dark matter from cosmos |url=http://apnews.excite.com/article/20130403/DA5E6JAG3.html |date=3 April 2013 |agency=Associated Press |access-date=3 April 2013}}</ref><ref name="BBC-20130403">{{cite news |last=Amos |first=Jonathan |title=Alpha Magnetic Spectrometer zeroes in on dark matter |url=https://www.bbc.co.uk/news/science-environment-22016504 |date=3 April 2013 |work=BBC |access-date=3 April 2013}}</ref><ref name="NASA-20130403">{{cite web |last1=Perrotto |first1=Trent J. |last2=Byerly |first2=Josh |title=NASA TV Briefing Discusses Alpha Magnetic Spectrometer Results |url=http://www.nasa.gov/home/hqnews/2013/apr/HQ_M13-054_AMS_Findings_Briefing.html |date=2 April 2013 |website=NASA |access-date=3 April 2013}}</ref><ref name="NYT-20130403">{{cite news |last=Overbye |first=Dennis |title=New Clues to the Mystery of Dark Matter |url=https://www.nytimes.com/2013/04/04/science/space/new-clues-to-the-mystery-of-dark-matter.html |archive-url=https://ghostarchive.org/archive/20220101/https://www.nytimes.com/2013/04/04/science/space/new-clues-to-the-mystery-of-dark-matter.html |archive-date=2022-01-01 |url-access=limited |date=3 April 2013 |work=The New York Times |access-date=3 April 2013}}{{cbignore}}</ref> === Collider searches for dark matter === An alternative approach to the detection of dark matter particles in nature is to produce them in a laboratory. Experiments with the [[Large Hadron Collider]] (LHC) may be able to detect dark matter particles produced in collisions of the LHC [[proton]] beams. Because a dark matter particle should have negligible interactions with normal visible matter, it may be detected indirectly as (large amounts of) missing energy and momentum that escape the detectors, provided other (non-negligible) collision products are detected.<ref name="kane watson">{{cite journal |author1=Kane, G. |author2=Watson, S. |title=Dark Matter and LHC:. what is the Connection? |journal=Modern Physics Letters A |year=2008 |volume=23 |pages=2103–2123 |doi=10.1142/S0217732308028314 |bibcode=2008MPLA...23.2103K |arxiv=0807.2244 |issue=26|s2cid=119286980 }}</ref> Constraints on dark matter also exist from the [[Large Electron–Positron Collider|LEP]] experiment using a similar principle, but probing the interaction of dark matter particles with electrons rather than quarks.<ref>{{cite journal |last1=Fox |first1=P.J. |last2=Harnik |first2=R. |last3=Kopp |first3=J. |last4=Tsai |first4=Y. |title=LEP Shines Light on Dark Matter |journal=Phys. Rev. D|year=2011 |volume=84 |issue=1 |page=014028 |doi=10.1103/PhysRevD.84.014028 |arxiv=1103.0240 |bibcode=2011PhRvD..84a4028F|s2cid=119226535 }}</ref> Any discovery from collider searches must be corroborated by discoveries in the indirect or direct detection sectors to prove that the particle discovered is, in fact, dark matter. == Alternative hypotheses == {{More|Alternatives to general relativity}} Because dark matter has not yet been identified, many other hypotheses have emerged aiming to explain the same observational phenomena without introducing a new unknown type of matter. The most common method is to modify general relativity. General relativity is well-tested on solar system scales, but its validity on galactic or cosmological scales has not been well proven.<ref>{{cite book | author = Peebles, P. J. E.| date= December 2004| title=Testing general relativity on the scales of cosmology| journal= General Relativity and Gravitation| arxiv= astro-ph/0410284|bibcode = 2005grg..conf..106P |doi = 10.1142/9789812701688_0010 <!--| loc = PROBING GENERAL RELATIVITY ON THE SCALES OF COSMOLOGY--> | isbn = 978-981-256-424-5 | pages = 106–117 | chapter= Probing General Relativity on the Scales of Cosmology| s2cid= 1700265}}</ref> A suitable modification to general relativity can in principle conceivably eliminate the need for dark matter. The best-known theories of this class are [[MOND]] and its relativistic generalization [[tensor–vector–scalar gravity]] (TeVeS),<ref>For a review, see: {{cite journal |title=The failures of the Standard Model of Cosmology require a new paradigm |author1=Kroupa, Pavel |display-authors=etal |journal=International Journal of Modern Physics D |date=December 2012 |volume=21 |issue=4 |page=1230003 |doi=10.1142/S0218271812300030 |arxiv=1301.3907 |bibcode=2012IJMPD..2130003K|s2cid=118461811 }}</ref> [[f(R) gravity]],<ref>For a review, see: {{cite journal |title=The dark matter problem from f(R) gravity viewpoint |author=Salvatore Capozziello |author2=Mariafelicia De Laurentis |journal=Annalen der Physik |date=October 2012 |volume=524 |issue=9–10 |page=545 |doi=10.1002/andp.201200109 |bibcode=2012AnP...524..545C |doi-access=free }}</ref> [[negative mass]], [[dark fluid]],<ref>{{cite web|url= https://www.ox.ac.uk/news/2018-12-05-bringing-balance-universe |title=Bringing balance to the Universe |publisher=University of Oxford}}</ref><ref>{{cite web|url= https://phys.org/news/2018-12-universe-theory-percent-cosmos.html |title=Bringing balance to the universe: New theory could explain missing 95 percent of the cosmos |publisher=Phys.Org}}</ref><ref name=Farnes>{{cite journal |last=Farnes |first=J.S. |title=A Unifying Theory of Dark Energy and Dark Matter: Negative Masses and Matter Creation within a Modified ΛCDM Framework |journal=Astronomy & Astrophysics |volume=620 |page=A92 |arxiv=1712.07962 |year=2018 |doi=10.1051/0004-6361/201832898 |bibcode=2018A&A...620A..92F |s2cid=53600834 }}</ref> and [[entropic gravity]].<ref name="physorgnewtheory">{{cite news |title=New theory of gravity might explain dark matter |url=https://phys.org/news/2016-11-theory-gravity-dark.html |website=phys.org |date=November 2016}}</ref> [[Alternatives to general relativity|Alternative theories]] abound.<ref>{{cite journal |title=Alternatives to dark matter and dark energy |first=Phillip D. |last=Mannheim |journal=Progress in Particle and Nuclear Physics |volume=56 |issue=2 |pages=340–445 |doi=10.1016/j.ppnp.2005.08.001 |arxiv=astro-ph/0505266 |date=April 2006 |bibcode=2006PrPNP..56..340M |s2cid=14024934 }}</ref><ref>{{cite journal |title=Beyond the Cosmological Standard Model |first1=Austin |last1=Joyce |display-authors=etal |journal=Physics Reports |date=March 2015 |volume=568 |pages=1–98 |doi=10.1016/j.physrep.2014.12.002 |arxiv=1407.0059 |bibcode=2015PhR...568....1J|s2cid=119187526 }}</ref> A problem with alternative hypotheses is that observational evidence for dark matter comes from so many independent approaches (see the "observational evidence" section above). Explaining any individual observation is possible but explaining all of them in the absence of dark matter is very difficult. Nonetheless, there have been some scattered successes for alternative hypotheses, such as a 2016 test of gravitational lensing in entropic gravity<ref>{{cite news |url=http://phys.org/news/2016-12-verlinde-theory-gravity.html |title=Verlinde's new theory of gravity passes first test |date=16 December 2016}}</ref><ref>{{cite journal |title=First test of Verlinde's theory of Emergent Gravity using Weak Gravitational Lensing measurements |first1=Margot M. |last1=Brouwer |display-authors=etal |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=466 |issue=3 |date=April 2017 |doi=10.1093/mnras/stw3192 |arxiv=1612.03034 |pages=2547–2559 |bibcode=2017MNRAS.466.2547B|s2cid=18916375 }}</ref><ref>{{cite web |url=https://www.newscientist.com/article/2116446-first-test-of-rival-to-einsteins-gravity-kills-off-dark-matter/ |title=First test of rival to Einstein's gravity kills off dark matter |date=15 December 2016 |access-date=20 February 2017}}</ref> and a 2020 measurement of a unique MOND effect.<ref>{{cite web|url=https://www.sciencedaily.com/releases/2020/12/201216155158.htm|title=Unique prediction of 'modified gravity' challenges dark matter|publisher=ScienceDaily|date=16 December 2020|access-date=14 January 2021}}</ref><ref>{{cite journal|title=Testing the Strong Equivalence Principle: Detection of the External Field Effect in Rotationally Supported Galaxies|first1=Kyu-Hyun|last1=Chae|display-authors=et al|journal=[[Astrophysical Journal]]|volume=904|date=20 November 2020|issue=1|page=51|doi=10.3847/1538-4357/abbb96|arxiv=2009.11525|bibcode=2020ApJ...904...51C|s2cid=221879077}}</ref> The prevailing opinion among most astrophysicists is that while modifications to general relativity can conceivably explain part of the observational evidence, there is probably enough data to conclude there must be some form of dark matter present in the Universe.<ref>{{cite web |url=http://www.preposterousuniverse.com/blog/2012/05/09/dark-matter-vs-modified-gravity-a-trialogue/ |title=Dark matter vs. modified gravity: A trialogue |author-link=Sean M. Carroll |author=Sean Carroll |date=9 May 2012 |access-date=14 February 2017}}</ref> == In popular culture == Dark matter regularly appears as a topic in hybrid periodicals that cover both factual scientific topics and science fiction,<ref>{{Cite journal |last=Cramer |first=John G. |date=2003-07-01 |title=LSST-The Dark Matter Telescope |journal=[[Analog Science Fiction and Fact]] |volume=123 |issue=7/8 |page=96 |issn=1059-2113 |id={{ProQuest|215342129}}}} (Registration required)</ref> and dark matter itself has been referred to as "the stuff of science fiction".<ref>{{Cite news |last=Ahern |first=James |date=2003-02-16 |title=Space Travel: Outdated Goal |page=O.02 |work=[[The Record (Bergen County)|The Record]] |id={{ProQuest|425551312}}}} (Registration required)</ref> Mention of dark matter is made in works of fiction. In such cases, it is usually attributed extraordinary physical or magical properties, thus becoming inconsistent with the hypothesized properties of dark matter in physics and cosmology. For example, dark matter serves as a plot device in the ''[[The X-Files|X-Files]]'' episode "[[Soft Light (The X-Files)|Soft Light]]",<ref>{{cite journal|first=Grace |last=Halden |title=Incandescent: Light Bulbs and Conspiracies |journal=Dandelion: Postgraduate Arts Journal and Research Network |date=Spring 2015 |volume=5 |number=2 |doi=10.16995/ddl.318}}</ref> in a manner that one reviewer found reliant upon the audience's ignorance.<ref>{{cite web|url=https://www.avclub.com/the-x-files-millennium-f-emasculata-soft-light-o-1798165326 |title=The X-Files/Millennium: "F. Emasculata"/"Soft Light"/"Our Town"/"Anasazi" |website=[[The AV Club]] |first=Zack |last=Handlen |date=2010-06-27 |access-date=2022-12-11}}</ref> A dark-matter-inspired substance known as "Dust" features prominently in [[Philip Pullman]]'s ''[[His Dark Materials]]'' trilogy,<ref>{{cite book|title=The Science of Philip Pullman's His Dark Materials |first1=Mary |last1=Gribbin |first2=John |last2=Gribbin |publisher=Random House Children's Books |year=2007 |isbn=978-0-375-83146-1 |pages=15–30}}</ref> and beings made of dark matter are antagonists in [[Stephen Baxter (author)|Stephen Baxter]]'s [[Xeelee Sequence]].<ref>{{cite journal|first=Andrew |last=Fraknoi |title=Science fiction for scientists |journal=[[Nature Physics]] |volume=12 |pages=819–820 |year=2019 |issue=9 |url=https://www.nature.com/articles/nphys3873 |doi=10.1038/nphys3873}}</ref> More broadly, the phrase "dark matter" is used metaphorically to evoke the unseen or invisible.<ref>{{cite web|first=Adam |last=Frank |author-link=Adam Frank |title=Dark Matter is in Our DNA |url=https://nautil.us/dark-matter-is-in-our-dna-2-236435/ |website=[[Nautilus Quarterly]] |date=2017-02-09 |access-date=2022-12-11}}</ref> <!-- Please do not add trivia about dark matter in popular culture to this section. Examples require secondary sources that provide context and indicate how those examples are significant. --> ==Gallery== {{Gallery |File:COSMOSDMmap2007.jpg|DM map by the [[Cosmic Evolution Survey]] (COSMOS) using the [[Hubble Space Telescope]] (2007).<ref>{{Cite web|title=First 3D map of the Universe's dark matter scaffolding|url=https://www.esa.int/Science_Exploration/Space_Science/First_3D_map_of_the_Universe_s_dark_matter_scaffolding|access-date=2021-11-23|website=www.esa.int|language=en}}</ref><ref>{{Cite journal|last1=Massey|first1=Richard|last2=Rhodes|first2=Jason|last3=Ellis|first3=Richard|last4=Scoville|first4=Nick|last5=Leauthaud|first5=Alexie|last6=Finoguenov|first6=Alexis|last7=Capak|first7=Peter|last8=Bacon|first8=David|last9=Aussel|first9=Hervé|last10=Kneib|first10=Jean-Paul|last11=Koekemoer|first11=Anton|date=January 2007|title=Dark matter maps reveal cosmic scaffolding|url=https://www.nature.com/articles/nature05497|journal=Nature|language=en|volume=445|issue=7125|pages=286–290|doi=10.1038/nature05497 |pmid=17206154|arxiv=astro-ph/0701594|bibcode=2007Natur.445..286M|s2cid=4429955|issn=1476-4687}}</ref> |File:CFHTLenSDMmap2012.jpg|DM map by the CFHT Lensing Survey (CFHTLenS) using the [[Canada–France–Hawaii Telescope]] (2012).<ref>{{Cite web|title=News CFHT - Astronomers reach new frontiers of dark matter|url=https://www.cfht.hawaii.edu/en/news/CFHTLens/|access-date=2021-11-26|website=www.cfht.hawaii.edu}}</ref><ref>{{Cite journal|last1=Heymans|first1=Catherine|last2=Van Waerbeke|first2=Ludovic|last3=Miller|first3=Lance|last4=Erben|first4=Thomas|last5=Hildebrandt|first5=Hendrik|last6=Hoekstra|first6=Henk|last7=Kitching|first7=Thomas D.|last8=Mellier|first8=Yannick|last9=Simon|first9=Patrick|last10=Bonnett|first10=Christopher|last11=Coupon|first11=Jean|date=2012-11-21|title=CFHTLenS: the Canada–France–Hawaii Telescope Lensing Survey: CFHTLenS|url=https://academic.oup.com/mnras/article-lookup/doi/10.1111/j.1365-2966.2012.21952.x|journal=Monthly Notices of the Royal Astronomical Society|language=en|volume=427|issue=1|pages=146–166|arxiv=1210.0032|doi=10.1111/j.1365-2966.2012.21952.x|s2cid=24731530}}</ref> (COSMOS map at the center) |File:KiDSDMmap2015.gif|DM map by the Kilo-Degree Survey (KiDS) using the [[VLT Survey Telescope]] (2015).<ref>{{Cite web|title=KiDS|url=https://kids.strw.leidenuniv.nl/pr_july2015.php|access-date=2021-11-27|website=kids.strw.leidenuniv.nl}}</ref><ref>{{Cite journal|last1=Kuijken|first1=Konrad|last2=Heymans|first2=Catherine|last3=Hildebrandt|first3=Hendrik|last4=Nakajima|first4=Reiko|last5=Erben|first5=Thomas|last6=Jong|first6=Jelte T. A.|last7=Viola|first7=Massimo|last8=Choi|first8=Ami|last9=Hoekstra|first9=Henk|last10=Miller|first10=Lance|last11=van Uitert|first11=Edo|date=10 October 2015|title=Gravitational lensing analysis of the Kilo-Degree Survey|url=https://academic.oup.com/mnras/article-lookup/doi/10.1093/mnras/stv2140|journal=Monthly Notices of the Royal Astronomical Society|language=en|volume=454|issue=4|pages=3500–3532|arxiv=1507.00738|doi=10.1093/mnras/stv2140|issn=0035-8711}}</ref> |File:HSCSDMmap2018.gif|DM map by the Hyper Suprime-Cam Survey (HSCS) using the [[Subaru Telescope]] (2018).<ref>{{Cite web|last=University|first=Carnegie Mellon|date=26 September 2018|title=Hyper Suprime-Cam Survey Maps Dark Matter in the Universe - News - Carnegie Mellon University|url=http://www.cmu.edu/news/stories/archives/2018/october/dark-matter-survey.html|url-status=live|website=www.cmu.edu|language=en|archive-url=https://web.archive.org/web/20200907194216/https://www.cmu.edu/news/stories/archives/2018/october/dark-matter-survey.html |archive-date=7 September 2020 }}</ref><ref>{{Cite journal|last1=Hikage|first1=Chiaki|last2=Oguri|first2=Masamune|last3=Hamana|first3=Takashi|last4=More|first4=Surhud|last5=Mandelbaum|first5=Rachel|last6=Takada|first6=Masahiro|last7=Köhlinger|first7=Fabian|last8=Miyatake|first8=Hironao|last9=Nishizawa|first9=Atsushi J|last10=Aihara|first10=Hiroaki|last11=Armstrong|first11=Robert|date=2019-04-01|title=Cosmology from cosmic shear power spectra with Subaru Hyper Suprime-Cam first-year data|url=https://academic.oup.com/pasj/article/doi/10.1093/pasj/psz010/5370019|journal=Publications of the Astronomical Society of Japan|language=en|volume=71|issue=2|pages=43|arxiv=1809.09148|doi=10.1093/pasj/psz010|issn=0004-6264}}</ref> |File:DESDMmap2021.png|DM map by the [[Dark Energy Survey]] (DES) using the [[Victor M. Blanco Telescope]] (2021).<ref>{{Cite journal|last1=Jeffrey|first1=N|last2=Gatti|first2=M|last3=Chang|first3=C|last4=Whiteway|first4=L|last5=Demirbozan|first5=U|last6=Kovacs|first6=A|last7=Pollina|first7=G|last8=Bacon|first8=D|last9=Hamaus|first9=N|last10=Kacprzak|first10=T|last11=Lahav|first11=O|date=2021-06-25|title=Dark Energy Survey Year 3 results: Curved-sky weak lensing mass map reconstruction|url=https://academic.oup.com/mnras/article/505/3/4626/6287258|journal=Monthly Notices of the Royal Astronomical Society|language=en|volume=505|issue=3|pages=4626–4645|arxiv=2105.13539|doi=10.1093/mnras/stab1495|issn=0035-8711}}</ref><ref>{{Cite journal|last=Castelvecchi|first=Davide|date=2021-05-28|title=The most detailed 3D map of the Universe ever made|url=http://www.nature.com/articles/d41586-021-01466-1|journal=Nature|language=en|pages=d41586–021–01466-1|doi=10.1038/d41586-021-01466-1|pmid=34050347|s2cid=235242965|issn=0028-0836}}</ref> }} == See also == {{columns-start}} {{column}} ;Related theories * {{annotated link|Dark energy}} * {{annotated link|Conformal gravity}} * [[Density wave theory]] – A theory in which waves of compressed gas, which move slower than the galaxy, maintain galaxy's structure * {{annotated link|Entropic gravity}} * {{annotated link|Dark radiation}} * {{annotated link|Massive gravity}} * {{annotated link|Unparticle physics}} ;Experiments * {{annotated link|DEAP}}, a search apparatus * {{annotated link|LZ experiment}}, large underground dark matter detector * {{annotated link|Dark Matter Particle Explorer|abbreviation=DAMPE}}, a space mission * {{annotated link|General antiparticle spectrometer}} * {{annotated link|MultiDark}}, a research program * {{annotated link|Illustris project}}, astrophysical simulations * {{annotated link|Future Circular Collider}}, a particle accelerator research infrastructure {{column}} ;Dark matter candidates * {{annotated link|Feebly Interacting Particles}} * {{annotated link|Light dark matter}} * {{annotated link|Mirror matter}} * {{annotated link|Exotic matter}} * {{annotated link|Neutralino#Relationship to dark matter|Neutralino}} * {{annotated link|Dark galaxy}} * {{annotated link|Scalar field dark matter}} * {{annotated link|Self-interacting dark matter}} * {{annotated link|Weakly interacting massive particles|abbreviation=WIMP}} * [[WISP (particle physics)|Weakly interacting slim particle]] (WISP){{snd}}Low-mass counterpart to WIMP * {{annotated link|Strongly interacting massive particle|abbreviation=SIMP}} * {{annotated link|Chameleon particle}} ;Other * {{annotated link|Galactic Center GeV excess}} {{columns-end}} == Notes == {{notelist|1}} == References == {{reflist}} ===Works cited=== * {{cite book |title=Dark Matter and the Dinosaurs: The Astounding Interconnectedness of the Universe |publisher=Ecco / Harper Collins Publishers |location=New York |first=Lisa |last=Randall |date=2015 |isbn=978-0-06-232847-2}} ==Further reading== * {{cite magazine |first1=Sabine |last1=Hossenfelder |author1-link=Sabine Hossenfelder |first2=Stacy S. |last2=McGaugh |author2-link=Stacy S. McGaugh |title=Is dark matter real? |magazine=[[Scientific American]] |volume=319 |issue=2 |pages=36–43 |date=August 2018}} == External links == {{Commons category|Dark matter}} * {{Curlie|Science/Astronomy/Cosmology/Dark_Matter}} * {{cite web |url=https://arstechnica.com/science/2017/02/a-history-of-dark-matter/ |title=A history of dark matter |date=February 2017 |website=[[Ars Technica]]}} * {{cite web |url=http://archive.cosmosmagazine.com/features/what-dark-matter/ |archive-url=https://web.archive.org/web/20151120074531/http://archive.cosmosmagazine.com/features/what-dark-matter/ |url-status=dead |archive-date=2015-11-20 |title=What is dark matter? |website=CosmosMagazine.com}} * [https://www.techexplorist.com/new-hubble-data-explains-missing-dark-matter/36391/ “Missing Dark Matter” in a far-away galaxy], Tech Explorer news item, from a 2020 ''Astrophysical Journal'' article, {{doi|10.3847/1538-4357/abc340}} * {{cite web |url=http://www.scilogs.com/the-dark-matter-crisis/2010/08/18/is-lambdacdm-or-standard-cosmology-a-4th-order-speculation-and-ought-it-be-further-researched/ |title=The Dark Matter Crisis |date=18 August 2010 |first=Pavel |last=Kroupa |department=posted in General |website=scilogs.com}} * {{cite web |url=http://www.hap-astroparticle.org/ |title=Helmholtz Alliance for Astroparticle Physics|date=26 January 2018}} * {{cite press release |url=http://www.nasa.gov/home/hqnews/2006/aug/HQ_06297_CHANDRA_Dark_Matter.html |title=NASA finds direct proof of dark matter |date=21 August 2006 |publisher=NASA}} * {{cite web |url=http://today.slac.stanford.edu/feature/darkmatter.asp |title=Dark Matter Observed |author=Tuttle, Kelen |date=22 August 2006 |publisher=SLAC ([[Stanford Linear Accelerator Center]]) Today}} * {{cite magazine |url=https://www.newscientist.com/article/dn7056-astronomers-claim-first-dark-galaxy-find/ |title=Astronomers claim first 'dark galaxy' find |date=23 February 2005 |magazine=[[New Scientist]]}} * {{cite news |url=https://www.theguardian.com/science/2009/dec/17/dark-matter-detected |title=Dark Matter Detected |date=17 December 2009 |newspaper=Guardian |location=London |first=Ian |last=Sample |access-date=1 May 2010}} * {{cite AV media |url=http://video.ias.edu/the-fifth-element |title=lecture on dark matter |author=Tremaine, Scott |publisher=IAS |medium=Video }} * {{cite web |url=https://www.sciencedaily.com/releases/2010/06/100613212708.htm |website=Science Daily |title=Astronomers' doubts about the Dark Side ...}} * {{cite web |last1=Gray |first1=Meghan |title=Dark Matter |url=http://www.sixtysymbols.com/videos/darkmatter.htm |website=Sixty Symbols |editor-link=Brady Haran |editor=Haran, Brady |publisher=[[University of Nottingham]] |author2=Merrifield, Mike |author3=Copeland, Ed |date=2010}} * {{cite journal |url=http://cosmos.nautil.us/short/144/the-physicist-who-denies-that-dark-matter-exists |title=The physicist who denies that dark matter exists |author=Carmeli, Oded |journal=Cosmos on Nautilus|date=27 February 2017 }} {{Dark matter}} {{Standard model of physics}} {{Cosmology topics}} {{Authority control}} {{Portal bar|Physics|Astronomy|Stars|Spaceflight|Outer space|Solar System|Science}} {{DEFAULTSORT:Dark Matter}} [[Category:Dark matter| ]] [[Category:Physical cosmology]] [[Category:Celestial mechanics]] [[Category:Large-scale structure of the cosmos]] [[Category:Physics beyond the Standard Model]] [[Category:Astroparticle physics]] [[Category:Exotic matter]] [[Category:Darkness|Matter]] [[Category:Theoretical physics]] [[Category:Concepts in astronomy]] [[Category:Unsolved problems in astronomy]] [[Category:Articles containing video clips]] [[Category:Dark concepts in astrophysics]]'