A phenomenon observed in a cluster of galaxies, whereby the cluster core loses energy via X-ray radiation because of the collisions between the gas particles. The radiation rate is proportional to the square of the density, and the cooling time, which remains in the outer parts too large, becomes smaller than the Hubble time in the core. As a result, the central regions of clusters of galaxies cool down; and since in the center of a cluster gas pressure and gravitational attraction are in equilibrium, the gas density has to rise to maintain the pressure necessary for supporting the outer layers of gas. To cause its density to rise, the cooled gas has to flow inward. As the densest gas, which cools quickest, is already concentrated in the center of the cluster, the inward flow will start at the center, soon followed by the outer layers. This flow of gas is called the cooling flow. Cooling flows are moderated through feedback due to the supermassive black hole in the nucleus of the central galaxy. The gas inflow to the center fuels the active galactic nucleus (AGN). The latter then heats again the gas through its radio jets.