What percentage of a galaxy cluster's mass is attributed to dark matter?

Around 80 to 90 percent

Galaxy clusters possess a mass budget heavily dominated by invisible components. While the visible galaxies are spectacular, they contribute only a small fraction to the total gravitational budget. The vast majority of the mass, typically estimated to fall between 80 and 90 percent of the total mass, is attributed to dark matter. The remainder of the mass budget is accounted for by the extremely hot, diffuse material known as the intracluster medium (ICM). This high proportion of dark matter is necessary to generate the immense gravitational field required to hold the cluster structure together against the internal motions of its galaxies and the thermal pressure of the hot gas.