Which of the following statements about rich clusters of galaxies (those with thousands of galaxies) is not true
They are sources of X-ray emission due to the presence of hot, intergalactic gas.
There likely have been numerous collisions among the member galaxies at some time in the past.
Galaxies in the central regions are predominantly spirals, while elliptical galaxies roam the outskirts.
They often have a very large, central dominant galaxy near their center, perhaps formed by the merger of s.everal individual galaxies.
The speeds of the galaxies in the cluster indicate that most of the cluster mass is dark matter.