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# Intergalactic space

Intergalactic space is the physical space between galaxies. Generally free of dust and debris, intergalactic space is very close to a total vacuum. Some theories put the average density of the Universe as the equivalent of one hydrogen atom per cubic meter. 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 than the Universe's average. The temperature is only 2.73 Kelvin. NASA's COBE mission (COsmic Background Explorer) measured the temperature as 2.725 +/- 0.002 K

Surrounding and stretching between galaxies, there is a rarefied plasma that is thought to possess a cosmic filamentary structure and that is slightly denser than the average density in the Universe. This material is called the intergalactic medium (IGM) and is mostly ionized hydrogen, i.e. a plasma consisting of equal numbers of electrons and protons. The IGM is thought to exist at a density of 10 to 100 times the average density of the Universe (10 to 100 hydrogen atoms per cubic meter). It reaches densities as high as 1000 times the average density of the Universe in rich clusters of galaxies.

The reason the IGM is thought to be mostly ionized gas is that its temperature is thought to be quite high by terrestrial standards (though some parts of it are only "warm" by astrophysical standards). As gas falls into the Intergalactic Medium from the voids, it heats up to temperatures of $10^5$ K to $10^7$ K, which is high enough for the bound electrons to escape from the hydrogen nuclei upon collisions. At these temperatures, it is called the Warm-Hot Intergalactic Medium (WHIM). Computer simulations indicate that on the order of half 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 $10^8$ K and above.