[1] This should happen as the ICM (a plasma) is quickly losing its energy by the emission of X-rays.
The X-ray brightness of the ICM is proportional to the square of its density, which rises steeply towards the centres of many clusters.
The typical [predicted] timescale for the ICM to cool is relatively short, less than a billion years.
In a steady state, the rate of mass deposition, i.e. the rate at which the plasma cools, is given by where L is the bolometric (i.e. over the entire spectrum) luminosity of the cooling region, T is its temperature, k is the Boltzmann constant and μm is the mean molecular mass.
Theories for why there is little evidence of cooling include[3] Heating by AGN is the most popular explanation, as they emit a lot of energy over their lifetimes, and some of the alternatives listed have theoretical problems.