galaxies are a vast and diverse subject of study by both professional astronomers and laymen alike. Although astronomers cannot agree on an exact number, it is believed that there are approximately 100 billion galaxies. In comparison, our galaxy, the Milky Way, is estimated to contain approximately half that number. If you were to divide this out into the nearby spiral systems, you would find about twice as many. galaxies also tend to cluster together, and if a galaxy has two or more satellites, it is likely that there are at least that many within its system.
It is possible for a galaxy to collapse, however, there are no signs of these happening within our current model of the Universe. Most astronomers believe that a collapse occurs when a huge galaxy star or planet falls onto it, or perhaps a collision between two very large space debris. It is not uncommon for super giant black holes to form at the centers of galaxies as well. These black holes can quickly consume a small satellite, or they may consume the gas that was spiraling around the black hole at a fraction of the speed of the matter itself. Other possibilities include collisions between smaller stars, which can cause the creation of a super cloud, or collisions between clouds of gas moving together.
It is impossible to measure the composition of a galaxy, but by using a technique called a gravitational microlensing, astronomers have been able to determine a great deal about the makeup of very compact clusters of galaxies. The resulting study, published in 2021 in Science Advances, showed that about two-thirds of all large galaxy clusters contain either a normal spiral mass or an irregular disk of dust. This dust contains relatively few normal stars, but it is composed mostly of dust. This type of galaxy, called a lentigo, consists mostly of elliptical disks and very heavy gravity, which are much like our own. However, a third type, called a bulge galaxy, consists mainly of large spiral arms and very little dust, which makes it much less compact than other types.