|Matter that has no radiation and therefore cannot be detected directly, but whose
presence can be inferred from dynamical phenomena produced by its gravitational influence.
The existence of dark matter is deduced mainly from the rotational speeds of galaxies,
velocities of galaxies in clusters, gravitational lensing by galaxy clusters, and
the temperature distribution of hot gas in galaxies and clusters of galaxies. Dark
matter plays also a central role in cosmic structure formation. There exists a large
number of non-baryonic dark matter candidates. They include, the hypothetical stable
particles WIMPs, neutralinos, axions, gravitinos, etc. Among unstable candidates are
gravitinos with mild R-parity violation and sterile neutrinos.