Dark matter

Dark matter is a form of matter that interacts gravitationally but emits, absorbs, and reflects no detectable electromagnetic radiation. Its presence is inferred entirely from its gravity: the fast motions of galaxies in clusters (Fritz Zwicky, 1933), the flat rotation curves of spiral galaxies (Vera Rubin, 1970s), gravitational lensing maps, and the acoustic-peak structure of the cosmic microwave background all require several times more mass than the visible stars and gas supply.

The evidence indicates that dark matter is non-baryonic — not made of protons and neutrons — because big-bang nucleosynthesis and the CMB independently fix the baryon density far below the total matter density. It is also 'cold,' meaning it was moving slowly when cosmic structure formed, and very weakly interacting: it passes through ordinary matter and itself almost unimpeded, as dramatically demonstrated by the Bullet Cluster, where the dark mass sailed through a cluster collision while the ordinary gas was slowed and left behind.

What dark matter actually is remains unknown. Leading candidates include weakly interacting massive particles (WIMPs), axions, sterile neutrinos, and primordial black holes. Decades of direct-detection experiments in deep underground laboratories and searches at particle colliders have so far returned null results, excluding large regions of parameter space without identifying the particle. In the standard ΛCDM cosmology, dark matter makes up about 27% of the universe's total energy budget, roughly five times the ~5% in ordinary matter.

Zwicky coined the term in 1933 from the Coma cluster's mass discrepancy; the idea was largely ignored until Rubin and Ford's flat rotation curves in the 1970s made it unavoidable. The non-baryonic nature was established through nucleosynthesis and CMB constraints in the following decades.