If one looks at the velocities (measured through Doppler Shift) of galaxies in a cluster, again, the galaxies appear to be responding to a LOT more mass in the cluster than is actually seen.
Because the galaxies at the center of the cluster collide, they strip off material from their halos. Stars and gas have been found to lie completely outside of galaxies, settled into the centers of enormous clusters. However, this gas is still not enough material to explain the high velocities measured in cluster galaxies.