Zwicky and Rubin
In 1933 Swiss astronomer Fritz Zwicky calculated that galaxies in the Coma Cluster were moving too fast to be held together by the mass of their visible stars. He proposed invisible "dunkle Materie," dark matter, but the idea was largely ignored. In the 1970s Vera Rubin and Kent Ford measured how fast galaxies such as Andromeda rotate and found "flat rotation curves": stars at the edges orbit as fast as those near the center. Under Newtonian mechanics the outer stars should be slower, implying that galaxies sit inside vast unseen halos.
More evidence
Mapping mass through gravitational lensing, the bending of light predicted by general relativity, reveals far more mass than visible matter can account for. In the Bullet Cluster, reported in 2006, two colliding clusters left their hot gas in the middle while most of the mass sailed out to either side, strong evidence that unseen matter is real. Analyses of the cosmic microwave background by the Planck satellite put the universe at about 5% ordinary matter, 27% dark matter and 68% dark energy, which drives accelerating expansion. Dark matter is thought to have formed the scaffolding on which galaxies grew after the Big Bang.
Hunting for it
Ordinary objects like black holes or faint stars can't supply enough mass, so dark matter is likely an undiscovered particle. Candidates include weakly interacting massive particles (WIMPs), ultralight axions and sterile neutrinos, but neither underground detectors nor the LHC, which found the Higgs boson, has seen a signal. Alternatives such as Mordehai Milgrom's 1983 Modified Newtonian Dynamics (MOND) tweak the law of gravity instead, though they struggle to explain clusters and the cosmic background together. Gravitational-wave astronomy and large telescope surveys may offer new clues.
FAQ
- Are dark matter and dark energy the same?
- No. Dark matter pulls things together by gravity and binds galaxies; dark energy is the mysterious factor speeding up cosmic expansion.
- Is dark matter passing through me right now?
- If it's a particle, probably, but it barely interacts with ordinary matter, so you'd never notice.
Sources reviewed: 2026-10-26
References
- Zwicky, F. (1933). Die Rotverschiebung von extragalaktischen Nebeln. Helvetica Physica Acta, 6, 110–127.
- Rubin, V. C., & Ford, W. K. (1970). Rotation of the Andromeda Nebula from a spectroscopic survey of emission regions. Astrophysical Journal, 159, 379–403.
- Planck Collaboration. (2020). Planck 2018 results. VI. Cosmological parameters. Astronomy & Astrophysics, 641, A6.