The space between galaxies in a cluster is not empty. It is filled with extremely hot gas called the intracluster medium (ICM), heated to tens of millions of degrees by gravitational compression. This gas emits X-rays and actually contains more mass than all the galaxies in the cluster combined.

The ICM was a surprise when discovered. It revealed that the visible galaxies are just the visible fraction. In a typical cluster, galaxies make up only about 3-5% of the total mass. The hot gas accounts for roughly 12-15%.