What is "warm-hot intergalactic medium"

n. (context astronomy English) the relatively hot (warmer than warm, but cooler than hot) intergalactic medium found within a filament of galaxies (galaxy filament), wall of galaxies (galaxy wall), sheet of galaxies (galaxy sheet) or galaxy hypercluster (hypercluster, hypercluster of galaxies). This intergalactic gas is cooler than the X-ray emitting multi-million degree intracluster medium, but warmer than cool intergalactic gas clouds and distant intergalactic space gas far from structures.

The warm–hot intergalactic medium (WHIM) refers to a sparse, warm-to-hot (10 to 10 K) plasma that cosmologists believe to exist in the spaces between galaxies and to contain 40–50% of the baryons (that is, 'normal matter' which exists as plasma or as atoms and molecules, in contrast to dark matter) in the universe at the current epoch. Because of the high temperature of the medium, the expectation is that it is most easily observed from the ultraviolet and low energy X-ray emission.

Within the WHIM, gas shocks are created as a result of active galactic nuclei, along with the gravitationally-driven processes of merging and accretion. Part of the gravitational energy supplied by these effects is converted into thermal emissions of the matter by collisionless shock heating.

In May 2010 a giant reservoir of WHIM was detected by the Chandra X-ray Observatory lying along the wall shaped structure of galaxies ( Sculptor Wall) some 400 million light-years from Earth.