Glossary · Thiên văn học
Dark matter
Matter whose presence is inferred from its gravitational effects on stars, galaxies and light, but which has never been directly detected. It does not emit, absorb or reflect light, and what it is made of has not yet been determined.
All the evidence for dark matter is indirect. Stars at the outer edges of spiral galaxies move too fast to be held in place by the visible matter alone. Light from distant galaxies is bent by gravity (gravitational lensing) more strongly than the luminous matter can account for. Observations of colliding galaxy clusters, such as the Bullet Cluster, show mass distributed differently from ordinary matter.
According to estimates cited by NASA and CERN, dark matter makes up about 27% of the universe, compared with about 5% for ordinary matter. On these estimates, most of the matter in the universe is dark matter.
Many candidates have been proposed, including WIMPs (weakly interacting massive particles), axions and primordial black holes, but none has been confirmed. Dark matter is not the same as dark energy: dark matter is inferred from the gravity it exerts, while dark energy is the name for the unknown cause of the universe's accelerating expansion.