This explains the formation of spiral galaxies. To talk about these, we must first talk about clusters of galaxies. As we know, stars are not spread uniformly across the Universe. Rather, they occur in clusters star clusters and galaxies. Similarly, galaxies are not distributed uniformly they occur in large aggregates known as clusters of galaxies or galaxy clusters.
Galaxy clusters are simply large numbers of galaxies that are all orbiting around one another .
Another way to say that is that they are gravitationally bound . So just as the moon orbits around the earth, and it is gravitationally bound to the earth, so are clusters of galaxies gravitationally bound to one another. The interesting thing about galaxy clusters is that they are the largest known gravitationally bound systems in the Universe. Rich clusters of galaxies . They sometimes have a huge elliptical galaxy right in the center we will come back to this point later.