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intergalactic medium

A thin gas that fills some parts of the empty regions between galaxies. The distribution is non-uniform; filaments of intergalactic medium are separated by voids with much lower density. The main ingredient of the intergalactic medium is ionized hydrogen, in other words: a plasma consisting of an equal number of hydrogen nuclei (protons) and electrons. The average density of the intergalactic medium is estimated to be between ten and a hundred times that of the universe as a whole, corresponding to between ten and a hundred hydrogen atoms per cubic meter.