Material whose presence can be inferred from its effects on the motions of stars and galaxies, but which cannot be seen directly because it emits little or no radiation; also known as missing mass. Evidence for dark matter in spiral galaxies is provided by their rotation curves. The existence of dark matter in rich clusters of galaxies can be deduced from the motions of the member galaxies (see Virial Theorem). Additional evidence for the existence of dark matter comes from observations of distant supernovae and of the cosmic microwave background. The different observational techniques all yield a similar value for the amount of dark matter, which is about 50 times greater than the total amount of matter contained in stars (see Cosmology). Models of how elements such as helium were formed during the first few minutes after the Big Bang imply that only about 20% of the dark matter consists of so-called baryonic matter, the protons and neutrons that make up stars, galaxies, and our everyday world. The composition of the remaining 80% is unknown, although various possibilities, including hypothetical particles such as axions and photinos, have been suggested. See also Cold Dark Matter; Hot Dark Matter; Macho; Non-Baryonic Matter; Wimp
Subjects: Astronomy and Astrophysics.