A term used to describe matter that can be inferred to exist from its gravitational effects, but does not emit or absorb detectable amounts of light.
Evidence for Dark Matter
Observations of the rotational speed of spiral galaxies, the confinement of hot gas in galaxies and clusters of galaxies, the random motions of galaxies in clusters, the gravitational lensing of background objects, and the observed fluctuations in the cosmic microwave background radiation require the presence of additional gravity, which can be explained by the existence of dark matter.
Amount of Dark Matter
The evidence suggests that the mass of dark matter in galaxies, clusters of galaxies, and the universe as a whole is about five or six times greater than the mass of ordinary light-emitting matter that makes up stars, planets, gas and dust.
Alternatives to Dark Matter
One possibility, considered unlikely by most astrophysicists, is that a modification of the theory of gravity can explain the effect attributed to dark matter.
What is Dark Matter?
The nature of dark matter is unknown. A substantial body of evidence indicates that it cannot be baryonic matter, i.e., protons and neutrons. The favored model is that dark matter is mostly composed of exotic particles formed when the universe was a fraction of a second old. Such particles, which would require an extension of the so-called Standard Model of elementary particle physics, could be WIMPs (weakly interacting massive particles), or axions, or sterile neutrinos.Detection of Dark Matter Particles
Various types of experimental searches for dark matter candidates are being pursued by a number of investigators: the direct detection of dark matter particles using innovative new detectors; the detection of X-rays or gamma-rays from the decay or annihilation of dark matter particles; and the detection of dark matter particles created by colliding beams of high energy protons.