Dark matter is matter that does not emit, absorb or reflect light, yet its gravity shapes galaxies and the whole universe. Ordinary matter, everything made of atoms, is only about 5 percent of the universe’s total content. Dark matter accounts for roughly 27 percent, and dark energy the remaining 68 percent or so.
The evidence
Galaxy clusters (1930s). Fritz Zwicky noticed that galaxies in the Coma Cluster moved too fast to be held together by the visible matter alone. He proposed invisible “dunkle Materie”.
Galaxy rotation (1970s). Vera Rubin and Kent Ford measured how fast stars orbit in spiral galaxies. Stars far from the center moved as fast as those near it, instead of slowing down as expected. A large unseen mass must surround each galaxy.
Gravitational lensing. Mass bends light. Lensing by galaxy clusters shows far more mass than their stars and gas contain, and lets astronomers map where the dark matter is, see gravitational lensing.
The Bullet Cluster. In this collision of two clusters, the hot gas (most of the ordinary matter) was slowed down in the middle, while lensing shows most of the mass passed through. Many astronomers see this as strong evidence that dark matter is a real substance, not just a modification of gravity.
The cosmic microwave background. The pattern of tiny temperature variations in the universe’s oldest light matches models only if dark matter is included, see evidence for the Big Bang.
Structure formation. Without dark matter’s extra gravity, galaxies would not have had time to form, see how galaxies form.
What could it be?
- WIMPs (weakly interacting massive particles): long the favorite, but sensitive underground detectors have not found them yet.
- Axions: very light particles that experiments are searching for.
- Sterile neutrinos and other hypothetical particles.
- Primordial black holes formed in the early universe, now constrained by many observations.
Some physicists propose modified gravity theories such as MOND instead of dark matter. They explain some galaxy rotation curves well but struggle with clusters and the cosmic microwave background.
How scientists are searching
Underground detectors wait for rare collisions between dark matter particles and atoms. Particle accelerators such as the Large Hadron Collider try to create them. Telescopes look for signals of dark matter particles annihilating. Surveys such as ESA’s Euclid mission map dark matter across billions of light-years through weak lensing.
Dark matter should not be confused with dark energy, which drives the accelerating expansion of the universe, see the future of the universe.
