The warm–hot intergalactic medium (WHIM) refers to a sparse, warm-to-hot (10⁵ to 10⁷ 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.