<<Intergalactic space is the physical space between galaxies. The huge spaces between galaxy clusters are called the voids. Present estimates put the average energy density of the Universe at the equivalent of 5.9 protons per cubic meter, including dark energy, dark matter, and ordinary, baryonic matter, or atoms. The atoms account for only 4.6% of the total energy density, or a density of one proton per four cubic meters (~4 milligrams of hydrogen per square meter per million light years)
. The density of the universe, however, is clearly not uniform; it ranges from relatively high density in galaxies—including very high density in structures within galaxies, such as planets, stars, and black holes—to conditions in vast voids that have much lower density, at least in terms of visible matter.
Surrounding and stretching between galaxies, there is a rarefied plasma that is organized in a cosmic filamentary structure. This material is called the intergalactic medium (IGM). The density of the IGM is 5-200 times the average density of the Universe (i.e., a density of up to about one gram of hydrogen per square meter per million light years)
. It consists mostly of ionized hydrogen; i.e. a plasma consisting of equal numbers of electrons and protons. As gas falls into the intergalactic medium from the voids, it heats up to temperatures of 105
K to 107
K, which is high enough so that collisions between atoms have enough energy to cause the bound electrons to escape from the hydrogen nuclei; this is why the IGM is ionized. At these temperatures, it is called the warm–hot intergalactic medium (WHIM). (Although the gas is very hot by terrestrial standards, 105
K is often called "warm" in astrophysics.) Computer simulations and observations indicate that up to half of the atomic matter in the universe might exist in this warm-hot, rarefied state. When gas falls from the filamentary structures of the WHIM into the galaxy clusters at the intersections of the cosmic filaments, it can heat up even more, reaching temperatures of 108
K and above in the so-called intracluster medium.>>