Not only do we now understand the basic structure of our galaxy well, but also recent years have unveiled the fate of the Milky Way. While the universe is expanding and on large scales, objects are moving apart from each other, on smaller scales gravity and local motions can bring objects together. Astronomers have seen evidence of interactions, collisions, and mergers of galaxies in numerous dense groups and clusters of galaxies. In fact, galaxy mergers are common mainly near the dense center of a cluster, contributing to the growth of the most massive galaxy (called a cD galaxy), which lies at the cluster's heart. In such groups, driven by gravity, galaxies eat each other. Larger galaxies can become larger and larger over time, giving these collections of stars, gas, and dust the ability to grow into massive bodies far more massive than their original mass.

We do not live in a dense cluster of galaxies like the Hercules Cluster, the Virgo Cluster, or the Coma Cluster. But even in our loose and small group of galaxies we have experienced – and will experience – mergers. To understand why, you first need to understand the dynamics of our own galaxy group.

Just more than a decade after his discovery of a Cepheid variable star in the Andromeda “nebula,” since known as the Andromeda Galaxy (M31), Edwin P. Hubble defined the nearest galaxies to us as the Local Group of galaxies, explained in his 1936 work The Realm of the Nebulae. The first nine members Hubble identified in our little group were the Milky Way, the Large Magellanic Cloud, the Small Magellanic Cloud, the Andromeda Galaxy, M32, NGC 205, the Pinwheel Galaxy (M33), Barnard's Galaxy (NGC 6822), and IC 1613. He listed as possible members IC 10, IC 342, and NGC 6946. In 1936, Hubble knew of nine members and perhaps as many as twelve.

Nearly 80 years later, we know a vast amount more about the Local Group of galaxies. The Local Group contains at least 50 galaxies, and astronomers are discovering tiny Local Group galaxies every now and then. There may be more than 100 galaxies in the group, spanning a sphere of space some 10 million light-years across.