Galaxies are grouped into clusters, typically tens of millions of light years across, with hundreds or thousands of members. They are held together by their mutual gravitation, and each galaxy moves around the centre of gravity of the cluster. Their speed of movement shows that they experience a greater gravitational pull than can be accounted for by the observed galaxies. Some of this missing mass consists of dim galaxies too faint to be detectable and some in dark intergalactic gas which emits X ray that can be detected. But there must be much more “dark matter” than has so far been accounted for. Furthermore, current theories of the origin of the universe suggest that there must be a hundred times as much matter in the universe as astronomers have so far detected.
A typical galaxy will have had a collision or a near miss with another galaxy half a dozen times in its life so far. Pair of galaxies that are in the process of near collision can be observed now. One pair is called the Mice because of the streamers of stars, resembling mouse tails that they are pulling from each other; similar features in another pair have led to the galaxies being dubbed the Antennae.
One galaxy can score a direct hit on another and pass right through. The individual stars are much more widely spaced in relation to their diameters than galaxies are, and they do not collide. But gas clouds in the two galaxies can collide and generate intense radiation, thus spawning millions of new stars. Such objects are called “starburst’ galaxies. When the intruding galaxy has departed,