A cluster of galaxies looks different at optical and x-ray wavelengths. In the optical band, you see the galaxies themselves—typically about a hundred—in orbit about the cluster’s center of mass. In x rays, you can still see the biggest galaxies, but the smaller ones are lost against a bright background of diffuse plasma that fills intracluster space.
The plasma is a byproduct of cluster formation. In the early universe, vast volumes of dark matter and baryons collapsed under their own gravity to form infant clusters. Following the collapse, most of the baryonic matter ended up as shock-heated plasma. The rest, about a fifth, formed galaxies. Unreactive even with itself, the dark matter presumably lay idle, but it has always dominated the clusters’ gravitational potential. And the gravitational potential, through virialization, is what has heated the plasma in clusters to temperatures of 107 K and higher.
The plasma radiates in...
