Dark Matter Candidates: What Could Dark Matter Actually Be?

Something invisible holds galaxies together. Identifying the dark matter candidates behind it is one of physics’ biggest goals.

We can’t see it, can’t touch it, and can’t detect it with any conventional instrument. But we know it’s there because its gravity shapes everything we can observe: the rotation curves of galaxies, the bending of light around galaxy clusters, the large-scale structure of the universe.

Dark matter makes up approximately 27% of the universe’s total energy content: about five times the amount of ordinary matter. Every atom, every star, every planet, every person is built from what amounts to a minor impurity in a cosmos dominated by matter we cannot directly detect.

The nature of dark matter is one of the most pressing unsolved problems in physics and cosmology. Dozens of experiments worldwide are searching for it. No candidate has yet been confirmed.

The Evidence Is Overwhelming

Before diving into what dark matter might be, it’s worth establishing just how strong the evidence is that it exists.

Galaxy rotation curves: In a galaxy shaped like our Milky Way, Newton’s law of gravity predicts that stars farther from the center should orbit more slowly: just as outer planets orbit the Sun more slowly than inner ones. But observations by Vera Rubin and others in the 1970s showed that galaxy rotation curves are flat: stars at large radii orbit just as fast as stars closer in. This requires either extra invisible mass surrounding the galaxy (dark matter), or a modification to gravity itself.

Gravitational lensing: Einstein’s general relativity predicts that mass bends light. The amount of lensing observed around galaxy clusters greatly exceeds what the visible matter could produce. The “Bullet Cluster”: two galaxy clusters that have recently collided – provides particularly clean evidence: the visible hot gas (the bulk of ordinary matter) was slowed by the collision, while the dark matter (detected via gravitational lensing) passed straight through, creating a spatial separation between the two.

Cosmic microwave background: The detailed pattern of temperature fluctuations in the CMB, the afterglow of the Big Bang, depends on the relative amounts of ordinary matter, dark matter, and dark energy. The best-fit model from NASA requires approximately 5× more dark matter than ordinary matter.

Large-scale structure: The observed distribution of galaxies and galaxy clusters throughout the universe, the cosmic web, is consistent with simulations that include dark matter but inconsistent with simulations without it.

Whatever dark matter is, it must: interact gravitationally, be electrically neutral (it doesn’t interact electromagnetically), be slow-moving (“cold”: non-relativistic at the time of structure formation), be stable or very long-lived, and be abundant. No Standard Model particle fits all these requirements.

The Leading Dark Matter Candidates

WIMPs: The Long-Favored Candidate

For decades, the leading candidate for dark matter has been Weakly Interacting Massive Particles (WIMPs). WIMPs would be massive particles (roughly 1–1000 times the proton mass) that interact via the weak nuclear force and gravity but not electromagnetism.

WIMPs are theoretically attractive for a compelling reason: if you take a particle that interacts via the weak force and calculate how many would be produced in the hot early universe and how many would remain after the universe cooled (the so-called WIMP miracle), you get approximately the right dark matter abundance. This coincidence strongly suggested that weak-force-scale physics was the right place to look.

Supersymmetry (SUSY): the theoretical framework that predicts partner particles for every Standard Model particle – naturally provides several WIMP candidates, most notably the neutralino.

The problem: despite 30+ years of increasingly sensitive searches, WIMPs have not been found.

Direct detection experiments search for WIMPs scattering off nuclei in detectors placed deep underground to shield from cosmic rays. LUX-ZEPLIN (LZ), XENONnT, and PandaX-4T are multi-tonne liquid xenon detectors that have searched to extraordinary sensitivity. They have found nothing. A WIMP striking a xenon nucleus would produce a tiny flash of light and free electrons, which these detectors are designed to capture.

Collider searches at the LHC have searched for WIMP production in proton-proton collisions. No supersymmetric particles have been found.

Indirect detection experiments search for the products of WIMP annihilation in regions of high dark matter density (galactic centers, dwarf galaxies). The Fermi Gamma-ray Space Telescope and others have set increasingly stringent limits. No confirmed signal.

The null results from these searches have excluded much of the original WIMP parameter space. WIMPs remain possible, but the simplest WIMP candidates are increasingly constrained.

Axions: The Other Top Contender

Axions are hypothetical particles originally proposed in 1977 by Roberto Peccei and Helen Quinn to solve an unrelated problem in particle physics: the strong CP problem, why the strong nuclear force doesn’t violate the symmetry between matter and antimatter (CP symmetry) as it would appear to be allowed to.

The Peccei-Quinn solution introduces a new quantum field whose quantum, the axion, has very specific properties: it’s extremely light (a tiny fraction of the electron mass), it’s extremely weakly interacting, it couples to photons in the presence of a magnetic field, and it would be produced abundantly in the early universe.

If axions exist with the right mass and coupling, they would be produced non-thermally in the early universe (through the misalignment mechanism: the axion field began oscillating as the universe expanded, producing a dense, cold gas of axions) in exactly the right quantities to be dark matter. This is another coincidence that suggests axions might be the answer.

The search for axions uses a different strategy from WIMP searches: axion haloscopes: resonant microwave cavities embedded in powerful magnetic fields. In a strong magnetic field, dark matter axions would occasionally convert to microwave photons that can be detected. The ADMX (Axion Dark Matter eXperiment) and related experiments have begun probing the theoretically motivated mass range with the sensitivity needed to detect QCD axions.

No detection yet: but the experiments are just now reaching the required sensitivity, and the search is ongoing.

Sterile Neutrinos

Standard neutrinos are “active”, they interact via the weak force. Sterile neutrinos are hypothetical partners that interact only gravitationally – they couple to Standard Model particles only through mixing with the active neutrinos.

Sterile neutrinos with masses in the keV range (much lighter than WIMP candidates) are viable dark matter candidates. They would be “warm” dark matter: moving fast enough to suppress the formation of the very smallest structures, which might help explain observed discrepancies between WIMP-based cold dark matter predictions and the observed number of small galaxies.

Sterile neutrinos decaying could produce X-ray photons, providing a possible detection channel. An observed 3.5 keV X-ray line in galaxy cluster spectra, first reported in 2014, generated excitement as a possible sterile neutrino signal, but subsequent observations have been inconclusive.

Primordial Black Holes

Before exotic new particles, there is a more mundane (but extraordinary) possibility: dark matter could be primordial black holes (PBHs): black holes formed in the early universe from density fluctuations, before any stars existed.

If primordial black holes are abundant enough, they could account for some or all of the dark matter. They would be gravitational, electrically neutral, and stable (over cosmological timescales for sufficiently massive ones). A review in Nature summarizes the observational constraints.

The detection of gravitational waves from black hole mergers by LIGO reignited interest in PBHs. If many of the observed binary black hole mergers involve PBHs rather than stellar black holes, it could indicate a cosmological population.

However, constraints from microlensing surveys, CMB observations, and gravitational wave statistics have significantly restricted the mass range in which PBHs could be all of the dark matter. PBHs with masses around 20-100 solar masses, the range where LIGO detects mergers, are constrained to be a small fraction of total dark matter.

A window remains open for asteroid-mass PBHs (around 10¹⁷–10²² grams), which are challenging to constrain observationally.

SIDM, Fuzzy Dark Matter, and Other Alternatives

Self-Interacting Dark Matter (SIDM): Standard cold dark matter predicts “cuspy” density profiles at galaxy centers, dark matter densities that peak sharply toward the center. Observations suggest galaxy centers may have “cores” instead, flatter density profiles. SIDM, which interacts with itself via a dark force, could naturally produce cored profiles through dark matter collisions.

Fuzzy Dark Matter (Ultra-Light Axions): If dark matter consists of extremely light particles (masses around 10⁻²² eV: far lighter than the QCD axion), quantum mechanical wave effects on galactic scales would suppress structure formation below certain scales. This “fuzzy” behavior could explain the apparent deficit of small galaxies and satellites around the Milky Way compared to cold dark matter predictions.

Primordial magnetic monopoles, superheavy WIMPZILLAS, Q-balls, dark atoms: the particle physics community has been creative in proposing candidates. Each comes with its own theoretical motivation and detection strategy. For a deeper look at how physicists approach these exotic possibilities, see our guide on how black holes form.

Modified Gravity: An Alternative to Dark Matter?

Some physicists argue that the evidence for dark matter might instead indicate that our theory of gravity is wrong on galactic scales. MOND (Modified Newtonian Dynamics), proposed by Mordehai Milgrom in 1983, modifies Newton’s law at very low accelerations, precisely the regime where dark matter effects appear.

MOND successfully explains galaxy rotation curves without dark matter, and predicts the Tully-Fisher relation (a correlation between galaxy luminosity and rotation speed) more accurately than cold dark matter models.

But MOND faces serious problems:

• It doesn’t naturally explain galaxy cluster dynamics; clusters require dark matter even in MOND.
• The Bullet Cluster, where the mass concentration (from lensing) separates from the gas, is very hard to explain without actual dark matter.
• There is no satisfactory relativistic extension of MOND, though Tensor-Vector-Scalar gravity (TeVeS) and RMOND are attempts.

Most cosmologists view modified gravity as a less compelling option than dark matter particles, though the debate is ongoing.

Why the Search Continues

The null results from WIMP searches haven’t killed dark matter research. They have redirected it: toward axions, toward lighter and heavier candidates outside the original WIMP window, toward new detection strategies.

New experiments are coming online: CMB-S4 will measure CMB polarization with unprecedented precision, constraining dark matter models. The Vera Rubin Observatory (LSST) will begin survey operations in 2025, mapping galaxy weak lensing across the sky to probe dark matter structure. Next-generation direct detection experiments will push WIMP sensitivity to the neutrino floor, the point at which solar and atmospheric neutrinos produce a background that masks dark matter signals.

Dark matter remains the most robust evidence for physics beyond the Standard Model. Whatever it is, identifying it would be one of the greatest scientific discoveries in history: comparable to the discovery of the electron, revealing a new constituent of the universe that underlies all structure we see.

The hunt continues.

Sources

• Bertone, G., Hooper, D. & Silk, J. (2005). Particle dark matter: Evidence, candidates and constraints. Physics Reports, 405(5–6), 279–390.
• Clowe, D. et al. (2006). A Direct Empirical Proof of the Existence of Dark Matter. Astrophysical Journal Letters, 648(2), L109.
• Rubin, V.C. & Ford, W.K. (1970). Rotation of the Andromeda Nebula from a Spectroscopic Survey of Emission Regions. Astrophysical Journal, 159, 379.
• LUX-ZEPLIN Collaboration. (2022). First Dark Matter Search Results from LZ. Lawrence Berkeley National Laboratory.
• NASA. (2023). Dark Matter. NASA Science.

Further reading: Dark matter on Wikipedia