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Unsolved problem in physics:

What is dark matter? How was it generated?

(more unsolved problems in physics)

In astronomy, dark matter is a hypothetical form of matter that does not interact with light or other electromagnetic radiation. Dark matter is implied by gravitational effects which cannot be explained by general relativity unless more matter is present than can be observed. Such effects occur in the context of formation and evolution of galaxies,^[1] gravitational lensing,^[2] the observable universe's current structure, mass position in galactic collisions,^[3] the motion of galaxies within galaxy clusters, and cosmic microwave background anisotropies.

In the standard lambda-CDM model of cosmology, the mass–energy content of the universe is 5% ordinary matter, 26.8% dark matter, and 68.2% a form of energy known as dark energy.^[4]^[5]^[6]^[7] Thus, dark matter constitutes 85%^[a] of the total mass, while dark energy and dark matter constitute 95% of the total mass–energy content.^[8]^[9]^[10]^[11]

Dark matter is not known to interact with ordinary baryonic matter and radiation except through gravity, making it difficult to detect in the laboratory. The most prevalent explanation is that dark matter is some as-yet-undiscovered subatomic particle, such as either weakly interacting massive particles (WIMPs) or axions.^[12] The other main possibility is that dark matter is composed of primordial black holes.^[13]^[14]^[15]

Dark matter is classified as "cold", "warm", or "hot" according to velocity (more precisely, its free streaming length). Recent models have favored a cold dark matter scenario, in which structures emerge by the gradual accumulation of particles.

Although the astrophysics community generally accepts the existence of dark matter,^[16] a minority of 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. So far none of the proposed modified gravity theories can describe every piece of observational evidence at the same time, suggesting that even if gravity has to be modified, some form of dark matter will still be required.^[17]

History

Early history

The hypothesis of dark matter has an elaborate history.^[18]^[19] Wm. Thomson, Lord Kelvin, discussed the potential number of stars around the Sun in the appendices of a book based on a series of lectures given in 1884 in Baltimore.^[20]^[18] He inferred their density using the observed velocity dispersion of the stars near the Sun, assuming that the Sun was 20–100 million years old. He posed what would happen if there were a thousand million stars within 1 kiloparsec of the Sun (at which distance their parallax would be 1 milli-arcsecond). Kelvin concluded

Many of our supposed thousand million stars – perhaps a great majority of them – may be dark bodies.^[20]^[21]

In 1906, Poincaré^[22] used the French term [matière obscure] ("dark matter") in discussing Kelvin's work.^[22]^[21] He found that the amount of dark matter would need to be less than that of visible matter, incorrectly, it turns out.^[21]^[18]

The second to suggest the existence of dark matter using stellar velocities was Dutch astronomer Jacobus Kapteyn in 1922.^[23]^[24]

A publication from 1930 by Swedish astronomer Knut Lundmark points to him being the first to realise that the universe must contain much more mass than can be observed.^[25] Dutch radio astronomy pioneer Jan Oort also hypothesized the existence of dark matter in 1932.^[24]^[26]^[27] Oort was studying stellar motions in the 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 incorrect.^[28]

In 1933, Swiss astrophysicist Fritz Zwicky studied galaxy clusters while working at Cal Tech and made a similar inference.^[29]^[b]^[30] 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 gravitational attraction to hold the cluster together.^[31] Zwicky's estimates were off by more than an order of magnitude, mainly due to an obsolete value of the Hubble constant;^[32] the same calculation today shows a smaller fraction, using greater values for luminous mass. Nonetheless, Zwicky did correctly conclude from his calculation that most of the gravitational matter present was dark.^[21] However unlike modern theories, Zwicky considered "dark matter" to be non-luminous ordinary matter.^[18]^: III.A

Further indications of mass-to-light ratio anomalies came from measurements of galaxy rotation curves. In 1939, H.W. Babcock reported the rotation curve for the Andromeda nebula (now called the Andromeda Galaxy), which suggested the mass-to-luminosity ratio increases radially.^[33] He attributed it to either light absorption within the galaxy or modified dynamics in the outer portions of the spiral, rather than to unseen matter. Following 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, Oort discovered and wrote about the large non-visible halo of NGC 3115.^[34]

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.^[35]

1970s

The hypothesis of dark matter largely took root in the 1970s. Several different observations were synthesized to argue that galaxies should be surrounded by halos of unseen matter. In two papers that appeared in 1974, this conclusion was drawn in tandem by independent groups: in Princeton, New Jersey, U.S.A., by Jeremiah Ostriker, Jim Peebles, and Amos Yahil, and in Tartu, Estonia, by Jaan Einasto, Enn Saar, and Ants Kaasik.^[36]

One of the observations that served as evidence for the existence of galactic halos of dark matter was the shape of galaxy rotation curves. These observations were done in optical and radio astronomy. In optical astronomy, Vera Rubin and Kent Ford worked with a new spectrograph to measure the velocity curve of edge-on spiral galaxies with greater accuracy.^[37]^[38]^[39]

At the same time, radio astronomers were making use of new radio telescopes to map the 21 cm line of atomic hydrogen in nearby galaxies. The radial distribution of interstellar atomic hydrogen (H^$I$) often extends to much greater galactic distances than can be observed as collective starlight, expanding the sampled distances for rotation curves – and thus of the total mass distribution – to a new dynamical regime. Early mapping of Andromeda with the 300 foot telescope at Green Bank^[40] and the 250 foot dish at Jodrell Bank^[41] already showed the H^$I$ rotation curve did not trace the decline expected from Keplerian orbits.

As more sensitive receivers became available, Roberts & Whitehurst (1975)^[42] 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; that paper's Figure 16^[42] combines the optical data^[39] (the cluster of points at radii of less than 15 kpc with a single point further out) with the H^$I$ data between 20 and 30 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. Rogstad & Shostak (1972)^[43] 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.^[43] In 1978, Albert Bosma showed further evidence of flat rotation curves using data from the Westerbork Synthesis Radio Telescope.^[44]

By the late 1970s the existence of dark matter halos around galaxies was widely recognized as real, and became a major unsolved problem in astronomy.^[36]

1980–1990s

A stream of observations in the 1980–1990s supported the presence of dark matter. Persic, Salucci & Stel (1996) is notable for the investigation of 967 spirals.^[45] The evidence for dark matter also included gravitational lensing of background objects by galaxy clusters,^[46]^{(pp 14–16)} the temperature distribution of hot gas in galaxies and clusters, and the pattern of anisotropies in the cosmic microwave background.

According to the current consensus among cosmologists, dark matter is composed primarily of some type of not-yet-characterized subatomic particle.^[47]^[48] The search for this particle, by a variety of means, is one of the major efforts in particle physics.^[49]

Technical definition

In standard cosmological calculations, "matter" means any constituent of the universe whose energy density scales with the inverse cube of the scale factor, i.e., $ρ \propto a -3$ . This is in contrast to "radiation", which scales as the inverse fourth power of the scale factor $ρ \propto a -4$ , and a cosmological constant, which does not change with respect to $a$ ( $ρ \propto a 0$ ).^[50] The different scaling factors for matter and radiation are a consequence of radiation redshift. For example, after doubling the diameter of the observable Universe via cosmic expansion, the scale, $a$ , has doubled. The energy of the cosmic microwave background radiation has been halved (because the wavelength of each photon has doubled);^[51] the energy of ultra-relativistic particles, such as early-era standard-model neutrinos, is similarly halved.^[c] The cosmological constant, as an intrinsic property of space, has a constant energy density regardless of the volume under consideration.^[50]

In principle, "dark matter" means all components of the universe which are not visible but still obey $ρ \propto a -3$ . In practice, the term "dark matter" is often used to mean only the non-baryonic component of dark matter, i.e., excluding "missing baryons".^[52] Context will usually indicate which meaning is intended.

Observational evidence

Galaxy rotation curves

Animation of rotating disc galaxies. Dark matter – shown in red – is more concentrated near the center and it rotates more rapidly.

The arms of spiral galaxies rotate around their 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 the galaxy can be modelled as a point mass in the centre and test masses orbiting around it, similar to the Solar System.^[d] From Kepler's Third Law, it is expected that the rotation velocities will decrease with distance from the center, similar to the Solar System. This is not observed.^[53] Instead, the galaxy rotation curve remains flat or even increases 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 may be a lot of non-luminous matter (dark matter) in the outskirts of the galaxy.

Velocity dispersions

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^[54] do not match the predicted velocity dispersion from the observed mass distribution, even assuming complicated distributions of stellar orbits.^[55]

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-rays 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 lensing (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.^[56]

Gravitational lensing

One of the consequences of general relativity is the gravitational lens. Gravitational lensing occurs when massive objects between a source of light and the observer act as a lens to bend light from this source. Lensing does not depend on the properties of the mass; it only requires there to be a mass. The more massive an object, the more lensing is observed. An example is a cluster of galaxies lying between a more distant source such as a quasar and an observer. In this case, the galaxy cluster will lens the quasar.

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.^[57] By measuring the distortion geometry, the mass of the intervening cluster can be obtained. In the weak regime, lensing does not distort background galaxies into arcs, causing minute distortions instead. By examining the apparent shear deformation of the adjacent background galaxies, the mean distribution of dark matter can be characterized. The measured mass-to-light ratios correspond to dark matter densities predicted by other large-scale structure measurements.^[58]^[59]

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 CMB 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 locations of these peaks depend on cosmological parameters. Matching theory to data, therefore, constrains cosmological parameters.^[60]

The CMB anisotropy was first discovered by 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 in 2013–2015. The results support the Lambda-CDM model.^[61]^[62]

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,^[62] but difficult to reproduce with any competing model such as modified Newtonian dynamics (MOND).^[62]^[63]

Structure formation

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 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.^[65] 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.^[65]^[66]

Bullet Cluster

The Bullet Cluster is the result of a recent collision of two galaxy clusters. It is of particular note because the location of the center of mass as measured by gravitational lensing is different from the location of the center of mass of visible matter. This is difficult for modified gravity theories, which generally predict lensing around visible matter, to explain.^[67]^[68]^[69]^[70] Standard dark matter theory however has no issue: the hot, visible gas in each cluster would be cooled and slowed down by electromagnetic interactions, while dark matter (which does not interact electromagnetically) would not. This leads to the dark matter separating from the visible gas, producing the separate lensing peak as observed.^[71]

Type Ia supernova distance measurements

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.^[72] Data indicates the universe is expanding at an accelerating rate, the cause of which is usually ascribed to dark energy.^[73] Since observations indicate the universe is almost flat,^[74]^[75]^[76] it is expected the total energy density of everything in the universe should sum to 1 (Ω_tot ≈ 1). The measured dark energy density is Ω_Λ ≈ 0.690; the observed ordinary (baryonic) matter energy density is Ω_b ≈ 0.0482 and the energy density of radiation is negligible. This leaves a missing Ω_dm ≈ 0.258 which nonetheless behaves like matter (see technical definition section above) – dark matter.^[77]

Sky surveys and 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 percent) preference for pairs of galaxies to be separated by 147 Mpc, compared to those separated by 130–160 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.^[78] Combining the CMB observations with BAO measurements from galaxy redshift surveys provides a precise estimate of the Hubble constant and the average matter density in the Universe.^[79] The results support the Lambda-CDM model.

Redshift-space distortions

Large galaxy redshift surveys may be used to make a three-dimensional map of the galaxy distribution. These maps are slightly distorted because distances are estimated from observed redshifts; 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.^[80] Results are in agreement with the lambda-CDM model.

Lyman-alpha forest

In astronomical spectroscopy, the Lyman-alpha forest is the sum of the absorption lines arising from the Lyman-alpha transition of neutral hydrogen in the spectra of distant galaxies and quasars. Lyman-alpha forest observations can also constrain cosmological models.^[81] These constraints agree with those obtained from WMAP data.

Theoretical classifications

Composition

The identity of dark matter is unknown, but there are many hypotheses about what dark matter could consist of, as set out in the table below.

Some dark matter hypotheses
Light bosons	quantum chromodynamics axions
	axion-like particles
	fuzzy cold dark matter
neutrinos	Standard Model^[e]
neutrinos	sterile neutrinos
weak scale	supersymmetry
	extra dimensions
	little Higgs
	effective field theory
	simplified models
other particles	weakly interacting massive particle
	self-interacting dark matter
	atomic dark matter^[83]^[84]^[85]^[86]
strangelet^[87]
superfluid vacuum theory
dynamical dark matter^[88]
macroscopic	primordial black holes^[13]^[14]^[89]^[15]^[90]^[91]^[92]^[93]^[94]^[95]
	massive compact halo objects (MACHOs)
	macroscopic dark matter (Macros)
modified gravity (MOG)	modified Newtonian dynamics (MoND)
	tensor–vector–scalar gravity (TeVeS)
	entropic gravity

Fermi-LAT observations of dwarf galaxies provide new insights on dark matter.

Baryonic matter

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. Most of the ordinary matter familiar to astronomers, including planets, brown dwarfs, red dwarfs, visible stars, white dwarfs, neutron stars, and black holes, fall into this category.^[18]^[96] A black hole would ingest both baryonic and non-baryonic matter that comes close enough to its event horizon; afterwards, the distinction between the two is lost.^[97]

These massive objects 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.^[46]^: 286^[98]

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.^[99]^[100] Agreement with observed abundances requires that baryonic matter makes up between 4–5% of the universe's critical density. In contrast, large-scale structure and other observations indicate that the total matter density is about 30% of the critical density.^[77]
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 solar masses, which covers nearly all the plausible candidates.^[101]^[102]^[103]^[104]^[105]^[106]
Detailed analysis of the small irregularities (anisotropies) in the cosmic microwave background by WMAP and Planck indicate that around five-sixths of the total matter is in a form that only interacts significantly with ordinary matter or photons through gravitational effects.^[107]

Non-baryonic matter

There are two main candidates for non-baryonic dark matter: new hypothetical particles and primordial black holes.

Unlike baryonic matter, nonbaryonic particles do not contribute to the formation of the elements in the early universe (Big Bang nucleosynthesis)^[47] 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).^[82]

In 2015, the idea that dense dark matter was composed of primordial black holes made a comeback^[108] following results of gravitational wave measurements which detected the merger of intermediate-mass black holes. Black holes with about 30 solar masses are not predicted to form by either stellar collapse (typically less than 15 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 over 60% of dark matter.^[109] However, that study assumed a monochromatic distribution to represent the LIGO/Virgo mass range, which is inapplicable to the broadly platykurtic mass distribution suggested by subsequent James Webb Space Telescope observations.^[110]^[89]

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 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.^[111] Nonetheless, research and theories proposing dense dark matter accounts for dark matter continue as of 2018, including approaches to dark matter cooling,^[112]^[113] 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.^[114]

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.^[115]^[116]

Free streaming length

Dark matter can be divided into cold, warm, and hot categories.^[117] 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.^[118]^[119]^[120] 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]}

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;^{[clarification needed]} the latter is excluded by high-redshift galaxy observations.^[49]

Fluctuation spectrum effects

These categories also correspond to fluctuation spectrum effects ^{[further explanation needed]} and the interval following the Big Bang at which each type became non-relativistic. Davis et al. wrote in 1985:^[121]

Candidate particles can be grouped into three categories on the basis of their effect on the 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 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. Gravitinos and photinos 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.

— Davis, Efstathiou, Frenk, & White (1985)^[121]

Alternative definitions

Another approximate dividing line is warm dark matter became non-relativistic when the universe was approximately 1 year old and 1 millionth of its present size and in the radiation-dominated era (photons and neutrinos), with a photon temperature 2.7 million Kelvins. Standard physical cosmology gives the particle horizon size as $2ct$ (speed of light multiplied by time) in the radiation-dominated era, thus 2 light-years. A region of this size would expand to 2 million light-years today (absent structure formation). The actual FSL is approximately 5 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 million light-years (or 3 megaparsecs) today, around the size containing an average large galaxy.

The 2.7 million Kelvin photon temperature gives a typical photon energy of 250 electronvolt, thereby setting a typical mass scale for warm dark matter: particles much more massive than this, such as GeV–TeV mass 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 electronvolt, have FSLs much larger than a protogalaxy, thus qualifying them as hot.

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^[122] and Preon stars^[123]) or RAMBOs (such as clusters of brown dwarfs), to new particles such as WIMPs and axions.

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 supersymmetric models offer dark matter candidates in the form of the WIMPy Lightest Supersymmetric Particle (LSP).^[124] Separately, heavy sterile neutrinos exist in non-supersymmetric extensions to the standard model which explain the small neutrino mass through the seesaw mechanism.

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 ≈ 300 eV to 3000 eV.^{[citation needed]}

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 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

Hot dark matter consists of particles whose FSL is much larger than the size of a protogalaxy. The neutrino qualifies as such a particle. They were discovered independently, long before the hunt for dark matter: they were postulated in 1930, and detected in 1956. Neutrinos' mass is less than 10⁻⁶ that of an electron. Neutrinos interact with normal matter only via gravity and the 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 "weakly interacting slender particles" (WISPs), as opposed to WIMPs.

The three known flavours of neutrinos are the electron, muon, and tau. Neutrinos oscillate among the flavours as they move. It is hard to determine an exact upper bound on the collective average mass of the three neutrinos. For example, if the average neutrino mass were over 50 eV/c² (less than 10⁻⁵ of the mass of an electron), the universe would collapse.^[125] CMB data and other methods indicate that their average mass probably does not exceed 0.3 eV/c². Thus, observed neutrinos cannot explain dark matter.^[126]

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. Deep-field observations show instead that galaxies formed first, followed by clusters and superclusters as galaxies clump together.

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 planets, stars, or black holes. Historically, the answer has been it cannot,^[f]^[127]^[128]^[129] because of two factors:

It lacks an efficient means to lose energy^[127]: 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 diversity of interactions needed to form structures^[129]: 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 neutrinos and electromagnetic radiation through fusion when they become energetic enough. Protons and neutrons can bind via the strong interaction and then form atoms with electrons 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).

However, there are theories of atomic dark matter similar to normal matter that overcome these problems.^[86]

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.^[130]^[131] Many experiments aim to test this hypothesis. Although WIMPs have been the main search candidates,^[49] axions have drawn renewed attention, with the Axion Dark Matter Experiment (ADMX) searches for axions and many more planned in the future.^[132] 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.^[82]

Direct detection

Direct detection experiments aim to observe low-energy recoils (typically a few keVs) 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 light or phonons 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 rays is minimized. Examples of underground laboratories with direct detection experiments include the Stawell mine, the Soudan mine, the SNOLAB underground laboratory at 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 mK, detect the heat produced when a particle hits an atom in a crystal absorber such as germanium. Noble liquid detectors detect scintillation produced by a particle collision in liquid xenon or argon. Cryogenic detector experiments include such projects as CDMS, CRESST, EDELWEISS, and EURECA, while noble liquid experiments include LZ, XENON, DEAP, ArDM, WARP, 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 and PICASSO, which use alternative methods in their attempts to detect dark matter.

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.^[133] The DAMA/NaI and more recent DAMA/LIBRA experimental collaborations have detected an annual modulation in the rate of events in their detectors,^[134]^[135] 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^[136] and XENON100.^[137]

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.^[138]^[139]^[140]^[141] 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) may then be separated from background, which should be isotropic. Directional dark matter experiments include DMTPC, DRIFT, Newage and MIMAC.

Indirect detection

Video about the potential gamma-ray detection of dark matter annihilation around supermassive black holes. (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 centre of the Milky Way) two dark matter particles could annihilate to produce gamma rays or Standard Model particle–antiparticle pairs.^[143] 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, antiprotons or positrons emanating from high density regions in the Milky Way and other galaxies.^[144] 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.^[49]^[82]

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 neutrinos.^[145] Such a signal would be strong indirect proof of WIMP dark matter.^[49] High-energy neutrino telescopes such as AMANDA, IceCube and ANTARES are searching for this signal.^[46]^: 298 The detection by LIGO in 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.^[146]^[147]^[148]

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.^[149]

The Fermi Gamma-ray Space Telescope is searching for similar gamma rays.^[150] In 2009, an as yet unexplained surplus of gamma rays from the Milky Way's galactic center was found in Fermi data. This Galactic Center GeV excess might be due to dark matter annihilation or to a population of pulsars.^[151] In April 2012, an analysis of previously available data from Fermi's Large Area Telescope instrument produced statistical evidence of a 130 GeV signal in the gamma radiation coming from the center of the Milky Way.^[152] WIMP annihilation was seen as the most probable explanation.^[153]

At higher energies, ground-based gamma-ray telescopes have set limits on the annihilation of dark matter in dwarf spheroidal galaxies^[154] and in clusters of galaxies.^[155]

The PAMELA experiment (launched in 2006) detected excess positrons. They could be from dark matter annihilation or from pulsars. No excess antiprotons were observed.^[156]

In 2013, results from the Alpha Magnetic Spectrometer on the International Space Station indicated excess high-energy cosmic rays which could be due to dark matter annihilation.^[157]^[158]^[159]^[160]^[161]^[162]

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.^[163] Constraints on dark matter also exist from the LEP experiment using a similar principle, but probing the interaction of dark matter particles with electrons rather than quarks.^[164] 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

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 theory underpinning most observational evidence for dark matter, general relativity, is well-tested on Solar System scales, but its validity on galactic or cosmological scales has not been well proven.^[165] 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),^[166] f(R) gravity,^[167] negative mass, dark fluid,^[168]^[169]^[170] and entropic gravity.^[171] Alternative theories abound.^[172]^[173]

Primordial black holes are considered candidates for components of dark matter.^[94]^[92]^[174]^[175] Early constraints on primordial black holes as dark matter usually assumed most black holes would have similar or identical ("monochromatic") mass, which was disproven by LIGO/Virgo results.^[90]^[91]^[93] In 2024, a review by Bernard Carr and colleagues concluded that primordial black holes forming in the quantum chromodynamics epoch prior to 10^–5 seconds after the Big Bang can explain most observations attributed to dark matter. Such black hole formation would result in an extended mass distribution today, "with a number of distinct bumps, the most prominent one being at around one solar mass."^[13]

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^[176]^[177]^[178] and a 2020 measurement of a unique MOND effect.^[179]^[180]

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.^[17]

In popular culture

Dark matter regularly appears as a topic in hybrid periodicals that cover both factual scientific topics and science fiction,^[181] and dark matter itself has been referred to as "the stuff of science fiction".^[182]

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 1995 X-Files episode "Soft Light".^[183]
A dark-matter-inspired substance known as "Dust" features prominently in Philip Pullman's His Dark Materials trilogy.^[184]
Beings made of dark matter are antagonists in Stephen Baxter's Xeelee Sequence.^[185]

More broadly, the phrase "dark matter" is used metaphorically in fiction to evoke the unseen or invisible.^[186]

Gallery

DM map by the Cosmic Evolution Survey (COSMOS) using the Hubble Space Telescope (2007)^[187]^[188]
DM map by the CFHT Lensing Survey (CFHTLenS) using the Canada–France–Hawaii Telescope (2012)^[189]^[190] (COSMOS map at the center)
DM map by the Kilo-Degree Survey (KiDS) using the VLT Survey Telescope (2015)^[191]^[192]
DM map by the Hyper Suprime-Cam Survey (HSCS) using the Subaru Telescope (2018)^[193]^[194]
DM map by the Dark Energy Survey (DES) using the Víctor M. Blanco Telescope (2021)^[195]^[196]

Notes

^ Since dark energy does not count as matter, this is ⁠26.8/4.9 + 26.8⁠ = 0.845.
^ "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."^[29]^(p 125)

[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.]
^ However, in the modern cosmic era, this neutrino field has cooled and started to behave more like matter and less like radiation.
^ This is a consequence of the shell theorem and the observation that spiral galaxies are spherically symmetric to a large extent (in 2D).
^ The three neutrino types already observed are indeed abundant, and dark, and matter, but their individual masses are almost certainly too tiny to account for more than a small fraction of dark matter, due to limits derived from large-scale structure and high-redshift galaxies.^[82]
^ "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." — Buckley & Difranzo (2018)^[127]

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External links

Tremaine, Scott. Lecture on dark matter (Video). IAS.
Gray, Meghan; Merrifield, Mike; Copeland, Ed (2010). Haran, Brady (ed.). "Dark Matter". Sixty Symbols. University of Nottingham.

[8] Since dark energy does not count as matter, this is ⁠26.8/4.9 + 26.8⁠ = 0.845.

[31] "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."^[29]^(p 125)

[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.]

[54] However, in the modern cosmic era, this neutrino field has cooled and started to behave more like matter and less like radiation.

[56] This is a consequence of the shell theorem and the observation that spiral galaxies are spherically symmetric to a large extent (in 2D).

[87] The three neutrino types already observed are indeed abundant, and dark, and matter, but their individual masses are almost certainly too tiny to account for more than a small fraction of dark matter, due to limits derived from large-scale structure and high-redshift galaxies.^[82]

[133] "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." — Buckley & Difranzo (2018)^[127]

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v t e States of matter (list)
State	Solid Liquid Gas / Vapor Supercritical fluid Plasma
Low energy	Bose–Einstein condensate Fermionic condensate Degenerate matter Quantum Hall Rydberg matter Strange matter Superfluid Supersolid Photonic molecule
High energy	QCD matter Quark–gluon plasma Color-glass condensate
Other states	Colloid Crystal Liquid crystal Time crystal Quantum spin liquid Exotic matter Programmable matter Dark matter Antimatter Magnetically ordered Antiferromagnet Ferrimagnet Ferromagnet String-net liquid Superglass
Phase transitions	Boiling Boiling point Condensation Critical line Critical point Crystallization Deposition Evaporation Flash evaporation Freezing Chemical ionization Ionization Lambda point Melting Melting point Recombination Regelation Saturated fluid Sublimation Supercooling Triple point Vaporization Vitrification
Quantities	Enthalpy of fusion Enthalpy of sublimation Enthalpy of vaporization Latent heat Latent internal energy Trouton's rule Volatility
Concepts	Baryonic matter Binodal Compressed fluid Cooling curve Equation of state Leidenfrost effect Macroscopic quantum phenomena Mpemba effect Order and disorder (physics) Spinodal Superconductivity Superheated vapor Superheating Thermo-dielectric effect

v t e Cosmology
Background	Age of the universe Big Bang Chronology of the universe Universe Observable universe
History of cosmological theories	Discovery of cosmic microwave background History of the Big Bang theory Religious interpretations of the Big Bang Timeline of cosmological theories
Past universe	Cosmic microwave background Cosmic neutrino background Gravitational wave background Inflation Nucleosynthesis
Present universe	FLRW metric Friedmann equations Hubble's law Expansion of the universe Accelerating expansion Redshift
Future universe	Future of an expanding universe Ultimate fate of the universe
Components	Dark energy Dark fluid Dark matter Quintessence Lambda-CDM model
Structure formation	Galaxy filament Galaxy formation Large quasar group Large-scale structure Reionization Shape of the universe Structure formation
Experiments	2dF 6dF BOOMERanG COBE Illustris project Observational cosmology Planck SDSS WMAP
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