An Etymological Dictionary of Astronomy and Astrophysics

The space containing a cluster of galaxies filled with a tenuous gas of temperature 10⁵ to 10⁷ K and density 10^-6 to 10^-4 cm^-3.
WHIM has been continuously shock-heated during the process of structure formation. It is so highly ionized that it can only absorb or emit far-ultraviolet
and soft X-ray photons, primarily at spectral lines of highly ionized C, O, Ne, and Fe. WHIM is thought to be the main reservoir of missing baryons.

See also: → warm; → hot; → intergalactic medium.