The short answer
Dark matter is a form of matter that does not emit, absorb, or reflect light. We know it exists because of its gravitational effects on visible matter — galaxy rotation curves, gravitational lensing, and the large-scale structure of the universe cannot be explained without it. What it is made of remains one of the biggest open questions in physics.
The evidence: galaxy rotation curves
In a galaxy where all the mass is in visible stars, orbital speed should decrease with distance from the centre — just as planets in the outer Solar System orbit more slowly than Mercury. Instead, observations show that stars at the outer edges of galaxies orbit at roughly the same speed as stars near the centre. This flat rotation curve implies that there is a large invisible mass halo surrounding the visible disc, providing extra gravitational pull. Astronomer Vera Rubin systematically documented this across dozens of galaxies in the 1970s.
The evidence: gravitational lensing
Gravity bends light. When dark matter concentrations lie between us and distant galaxies, they bend the background light into arcs, rings, and multiple images. By mapping these distortions, astronomers can reconstruct the distribution of mass — visible and invisible — in galaxy clusters. The Bullet Cluster is the most famous example: a collision of two galaxy clusters where the dark matter (inferred from lensing) cleanly separated from the hot gas (observed in X-rays), providing compelling evidence that dark matter is distinct from ordinary matter.
What might dark matter be?
Weakly Interacting Massive Particles — the leading theoretical candidate. Would interact only via gravity and the weak force. Not detected despite decades of sensitive experiments.
Ultra-light particles predicted by QCD theory. Could explain dark matter if they exist. Several experiments (ADMX, HAYSTAC) are searching for axion-photon conversion in strong magnetic fields.
A hypothetical heavier cousin of the neutrino that interacts only via gravity. Some X-ray telescope observations have hinted at a possible signal but results remain inconclusive.
Black holes formed in the early universe (not from stellar collapse). LIGO gravitational wave observations constrain but do not rule out a contribution to dark matter. See our black hole guide.
The Euclid space telescope
Launched in 2023, the ESA Euclid mission is mapping the large-scale structure of the universe with unprecedented precision. By measuring how galaxies cluster and how dark matter distorts their shapes via weak lensing across billions of galaxies, Euclid aims to constrain the nature of both dark matter and dark energy simultaneously. Read the latest Euclid findings in our news section.