The intergalactic medium is the hot, X-ray emitting gas that permeates the space between galaxies. With temperatures of millions of degrees Kelvin and containing less than one atom per cubic meter (a density less than 10^-27 kg/m³), intergalactic space is one of the hottest and most rarefied environments in the Universe.

Originally it was assumed that the intergalactic medium was composed entirely of primordial hydrogen and helium left over from the Big Bang. However, in the 1970s, X-ray observations revealed large quantities of metals mixed in with the hydrogen and helium. These metals could only have been made by stars within the galaxies, and somehow later ejected into intergalactic space.

Astronomers now believe that the intergalactic medium is enriched through the action of galactic winds. Thought to be driven by either starbursts or active galactic nuclei, these are streams of highly-charged particles (including metals) that are often seen blowing out of galaxies.

See also:chemical evolution.