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Cooling flow refers to an inward flow of intracluster medium (ICM) (plasma) within a galaxy cluster due to temperature and pressure drops at the center. The central ICM is clearly cooling: its observed X-ray emission transports the energy of its heat out of the cluster, and the inward flow of ICM is expected. A problem (called the cooling flow problem) is that some galaxy clusters are transferring energy sufficiently fast by X-ray emission that their ICM should be cooler by now. The question of what replenishes the heat is of research interest.