Published online by Cambridge University Press: 09 March 2009
On the average, intergalactic space contains a component of gas, dust, and plasma. The numerical density of this medium depends very strongly on what the actual distance is to objects of intermediate and high redshifts. The average density could be enormously higher than currently assumed. In addition, there is the problem of how the material outside the galaxies is distributed. X-ray emission observed in clouds exterior to galaxies as well as in diffuse backgrounds in clusters of galaxies also attest to the radiation of hot plasmas between galaxies. The sizes of these X-ray emitting regions scale with their distances.