On the largest scales the universe is expanding: galaxies move apart from each other. But galaxies close and massive enough can overcome that expansion through their mutual gravity and refuse to fly apart. Such collections are called gravitationally 'bound' structures. Our own galactic neighborhood — the Milky Way, the Andromeda Galaxy (M31), the Triangulum Galaxy (M33), and dozens of dwarf companions — spans roughly 10 million light-years and is one of these bound islands: the Local Group.

The individual galaxies are far older; the Milky Way's first stars date to 13.4 Gya, Andromeda about the same. But their binding as a group — the moment when local gravity took over from Hubble expansion and they began drifting toward each other — was later. Models place this threshold somewhere between 5 and 7 Gya, when the universe was about two-thirds its present age. The acceleration of expansion driven by dark energy switched on around 5 Gya; after that, the formation of new bound structures effectively stopped. The Local Group is among the last gravitationally bound systems the universe will ever assemble.

The Local Group's two giants, the Milky Way and Andromeda, are now closing at about 110 km per second. In some 4–5 billion years — a time scale comparable to the Sun's remaining life — they will collide. After the collision, the two spirals will merge into a single giant elliptical galaxy that astronomers jokingly call 'Milkomeda.' Stellar densities are so low that individual stars rarely collide during such mergers; but where gas clouds compress, intense bursts of new star formation will erupt.

The Local Group will remain the universe's unit that bears most directly on us. More distant galaxies are receding through accelerating expansion; in 100 billion years almost everything outside the Local Group will have slipped out of our observable horizon. The only place we will see permanently is our own galactic island.