Dark matter

A cluster of galaxies bends the light behind it. The bending reveals more mass than its stars and gas can supply. What is the unseen part made of?[1] [2]

Under standard gravity, galaxy motions and bent light point to more mass than known stars and gas can supply. What is this unseen matter made of, and how does it interact with ordinary matter? Locating its gravitational pull does not identify its ingredients.

The dwarf galaxy UGCA 442 turns faster than its catalogued ordinary matter predicts under Newtonian gravity. The SPARC catalog gives a rotation speed of 57.8 km/s at 5.48 kiloparsecs from its center. At that radius, the gas contribution is 18.73 km/s and the tabulated stellar-disk contribution is 12.04 km/s, with no bulge contribution.

The disk entry assumes a stellar mass-to-light ratio of 1 in solar units at 3.6 micrometers. Choosing 0.5 instead halves the disk’s mass contribution; it scales the squared speed, rather than halving the speed itself. Write $v$ for the predicted ordinary-matter speed, $v_g$ for the gas contribution and $v_d$ for the disk entry at a mass-to-light ratio of 1. Using speeds in km/s,

$$\begin{gathered}v=\sqrt{v_g^2+0.5v_d^2}\\=\sqrt{18.73^2+0.5(12.04)^2}\,\mathrm{km/s}\\\approx20.6\,\mathrm{km/s}.\end{gathered}$$

The gap motivates an additional mass component under these assumptions. Its size depends on the stellar mass estimate, distance and the galaxy’s inclination. This calculation is one worked catalog example, rather than a determination of the missing component’s ingredients.

Gravitational lensing offers another measurement: correlated distortions of background galaxies constrain a projected mass distribution. Material at different distances can contribute along the line of sight. Comparing the reconstructed map with starlight and X-ray gas tests whether ordinary matter can supply the inferred gravity; it does not specify the missing component’s physical identity.[1] [2] [3] [5] [6] [9]

We can map its gravitational pull without knowing its ingredients. Proposed particles remain unconfirmed; a sharper map does not name the substance.[1] [8]

The cosmic microwave background—the light left from the early universe—adds a different constraint. In the standard cosmological model, its temperature and polarization patterns distinguish the densities of ordinary matter and cold dark matter; adding more faint stars is not equivalent. A candidate must fit this cosmic abundance as well as galaxy and cluster observations. An alternative account of gravity also has to reproduce that combination.

Searches test particular interactions. A September 2026 LZ preprint reports one unusual xenon recoil and global $2.6\sigma$ tension with its background model after accounting for the many signal models tested. Here $\sigma$ expresses the significance in standard-deviation units; “global” includes that search across models, rather than quoting the most unusual individual fit. This does not identify a dark matter particle. Exclusion limits also apply to assumed masses and interactions, rather than to every possible constituent.[7] [8]

Most of the universe’s matter appears to be dark. Identifying it would tell us what supplies much of the gravity that helps galaxies form and holds them together.[1] [4] [7]

The aim is to connect two kinds of evidence. A detector signal could reveal how a candidate interacts with ordinary matter; astronomical observations show how the missing component gravitates and clusters. A proposed constituent must give a consistent account of both. One unexplained event cannot establish that connection.

Its identity would also make cosmic structure a test of particular physics. If dark matter particles scatter off one another, for example, cluster collisions may shift their mass relative to the galaxies. A 2025 Webb Bullet Cluster analysis uses this idea to constrain scattering, but describes its bound as illustrative because the merger history and the relationship between scattering and offsets are uncertain.[3] [8] [9]