Without the gravitational attraction of dark matter, pressure would expel the gas from the cluster. Deep images often reveal giant arcs, the gravitationally lensed images of background galaxies.
Dark matter is needed to produce the strong bending of light.
If the galaxy's stars provided all of its mass, we would expect the rotation speed to drop outside this radius. A galaxy's dark halo is much more extended than its light, and it contains most of the galaxy's mass. There is some evidence that elliptical galaxies have equally large dark halos, but it's harder to tell. Several forms have been proposed. The challenge is to package the baryons in a way that keeps them non-luminous.