Many galaxies in the universe live alongside others, tethered to each other by their mutual gravity, to form giant clusters; the largest gravitationally bound structures in the universe. The galaxies in a cluster are only a tiny fraction of the total matter present – there’s up to ten times as much mass in the form of a hot tenuous atmosphere filling the space between the galaxies, known as the intracluster medium.
The enormous gravitational field of the whole cluster – particularly that of the dominant but unobservable dark matter – squeezes this gas to heat it to extraordinarily high temperatures of millions of degrees. There is so much energy in this gas that the electrons are no longer bound
