In astronomy and cosmology, dark matter is a currently-undetermined type of matter hypothesized to account for a large part of the mass of the universe, but which neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly seen with telescopes. Dark matter is estimated to constitute 83% of the matter in the universe and 23% of the mass-energy.
Dark matter came to the attention of astrophysicists in recent decades due to discrepancies between the mass of large astronomical objects determined from their gravitational effects, and mass calculated from the "luminous matter" they contain; such as stars, gas and dust. It was first postulated by Jan Oort in 1932 to account for the orbital velocities of stars in the Milky Way and Fritz Zwicky in 1933 to account for evidence of "missing mass" in the orbital velocities of galaxies in clusters. Subsequently, other observations have indicated the presence of dark matter in the universe, including the rotational speeds of galaxies, gravitational lensing of background objects by galaxy clusters such as the Bullet Cluster, and the temperature distribution of hot gas in galaxies and clusters of galaxies. According to consensus among cosmologists, dark matter is believed to be composed primarily of a new, not yet characterized, type of subatomic particle. The search for this particle, by a variety of means, is one of the major efforts in particle physics today. Though the existence of dark matter is generally accepted by the mainstream scientific community, some alternative theories have been proposed to explain the anomalies that dark matter is intended to account for, without hypothesizing dark matter.