intergalactic medium

The intergalactic medium (IGM) is matter such as dust and gas between galaxies. Similar matter between stars within a galaxy is called the interstellar medium (ISM). The term IGM is often used for the medium between galaxies within a galaxy cluster, but the term intracluster medium (ICM) more specifically refers to that. IGM is often plasma in filaments, mostly ionized hydrogen. Some of it is hot, up to 10⁵ to 10⁷ K, and is termed the warm-hot intergalactic medium (WHIM) and observed in ultraviolet and low-energy X-ray radiation.