What connection does the author draw between the rotation of galaxy clusters and dark matter?

Galaxy clusters have several thousands of galaxies and dark matter is a cosmic phenomenon accounting for approximately 27 percent of the matter and energy but is mysterious. Galaxy clusters help in exploration of the dark matter as the scientists believe that the mass of the galaxy cluster is directly proportional to the dark matter environment that surrounds it. In other terms, the surrounding dark matter gives an indication of the packing of galaxies in clusters. The dark matter containing the gases filled between the galaxies are very hot and cannot be detected by visible light telescopes and may be detected using gamma rays or X-rays or by gravitational lensing.

c. would be the answer : )

explanation: