The material between the galaxies in a cluster of galaxies. The ICM contains several components. The presence of tenuous hot gas is revealed through its X-ray emission. It typically consists of only one atom in 1,000 cm3 but accounts for an estimated 10 per cent of the total cluster mass. Diffuse radio emission from clusters is probably synchrotron radiation produced by high-energy particles in the ICM. Stars that have been torn from galaxies by the gravitational interactions between them are also found in intergalactic space. These observed components, and the galaxies themselves, still account for only 20 per cent of the mass of a typical cluster judged by applying gravitational theory to the motion of member galaxies. Though some of this unseen mass is almost certainly associated with galaxies, the behaviour of clusters suggests that there is also unseen matter in the ICM, but its nature is unknown. See also: dark matter.