What is dark matter made of?
Most of the matter in the universe gives off no light and has never been caught by a detector. What is it?
The question
Galaxies and galaxy clusters move as if they contain far more mass than we can see in stars, gas and dust, and the oldest light in the universe carries the imprint of that extra mass too. Astronomers call it dark matter. Its gravity is measured routinely; what it is remains unknown.
What we know
- In 1933 Fritz Zwicky found that galaxies in the Coma cluster move too fast to be held together by the visible mass. From the 1970s, Vera Rubin, Kent Ford and others showed that the outer parts of spiral galaxies orbit about as fast as the inner parts, which requires mass far beyond the visible disc.
- Gravitational lensing, the bending of light by mass, maps where the mass is. In the “Bullet Cluster”, two colliding clusters, most of the mass has separated from most of the ordinary matter (hot gas) — hard to explain by changing the law of gravity alone.
- The cosmic microwave background, measured precisely by the Planck satellite, fits a model with roughly five times as much dark matter as ordinary matter. It must be “cold” (slow compared with light) and non-baryonic: not made of protons and neutrons.
- It interacts with ordinary matter very weakly, if at all, except through gravity. Neutrinos have this property and contribute a little, but they are too light and fast to explain how galaxies formed.
- Searches for dark compact objects in the Milky Way’s halo — faint stars, planets, black holes — rule them out as most of the dark matter over a wide range of masses, though some windows remain.
Main hypotheses
WIMPs
Weakly interacting massive particles, roughly as heavy as atoms, would have been produced in the early universe in about the right amount — the “WIMP miracle”. Detectors with tonnes of liquid xenon, deep underground, wait for one to knock into an atomic nucleus. So far they have seen nothing.
Axions
Extremely light particles first proposed to solve an unrelated puzzle about the strong nuclear force. In a strong magnetic field an axion could turn into a faint microwave photon; experiments such as ADMX tune a resonant cavity through frequencies, listening for it.
A dark sector
Dark matter could be one or several particles with forces of their own, barely connected to the particles we know. Many such possibilities are very hard to test.
Primordial black holes
Black holes formed in the first moments after the Big Bang could act as dark matter. Most mass ranges are now excluded by observations, but not all.
Modified gravity
Perhaps nothing is missing, and gravity behaves differently at very low accelerations (MOND). This describes the rotation of individual galaxies strikingly well, but struggles with clusters, lensing and the cosmic microwave background unless some dark matter is added anyway.
Where understanding ends
Everything we know about dark matter comes from its gravity: how much there is, where it is, how it clumps. We know no property of it beyond that — not whether it is made of particles, what they weigh, whether they interact with anything, or whether it is one thing or many. Serious candidates differ in mass by dozens of orders of magnitude, from particles far lighter than an electron to black holes as heavy as asteroids.
There are also tensions on small scales: some dwarf galaxies look different from what simulations of cold dark matter predict. This may point to new physics in dark matter, or only to an incomplete picture of how gas, stars and supernovae reshape galaxies.
Several claimed signals — an excess of gamma rays from the galactic centre, a yearly modulation in one underground detector — have not been independently confirmed or have ordinary explanations.
What would settle it
- A direct detection in an underground experiment, confirmed by another experiment with a different target.
- A new particle made at a collider whose properties match the astronomical measurements.
- A sharp, persistent signal at one frequency in an axion search.
- Astronomical evidence that dark matter interacts with itself or decays, for example in the shapes of small galaxies.
- Or a clear failure of dark-matter models where modified gravity succeeds; tests with wide pairs of stars have so far given conflicting results.
Sources
- Zwicky, F. (1933; English translation 2009). The redshift of extragalactic nebulae. General Relativity and Gravitation, 41, 207–224.
- Rubin, V. C., & Ford, W. K. Jr. (1970). Rotation of the Andromeda Nebula from a spectroscopic survey of emission regions. The Astrophysical Journal, 159, 379.
- Milgrom, M. (1983). A modification of the Newtonian dynamics as a possible alternative to the hidden mass hypothesis. The Astrophysical Journal, 270, 365.
- Clowe, D., et al. (2006). A direct empirical proof of the existence of dark matter. The Astrophysical Journal, 648, L109–L113.
- Bullock, J. S., & Boylan-Kolchin, M. (2017). Small-scale challenges to the ΛCDM paradigm. Annual Review of Astronomy and Astrophysics, 55, 343–387.
- Bertone, G., & Hooper, D. (2018). History of dark matter. Reviews of Modern Physics, 90, 045002.
- Du, N., et al. (ADMX Collaboration) (2018). Search for invisible axion dark matter with the Axion Dark Matter Experiment. Physical Review Letters, 120, 151301.
- Planck Collaboration (2020). Planck 2018 results. VI. Cosmological parameters. Astronomy & Astrophysics, 641, A6.
- Aalbers, J., et al. (LZ Collaboration) (2023). First dark matter search results from the LUX-ZEPLIN (LZ) experiment. Physical Review Letters, 131, 041002.