Mergers

Galaxies are like businesses: They grow bigger through mergers, with the big guys gobbling up the smaller ones.
Our home galaxy, the Milky Way, is a prime example. It’s an amalgam of many galaxies — perhaps hundreds of them. These mergers have pumped up the Milky Way to gigantic proportions. It spans a hundred thousand light-years or more, and consists of hundreds of billions of stars.
Astronomers have detected remnants of many of the smaller galaxies the Milky Way has gobbled up. They form streamers of stars that can stretch across thousands of light-years. The stars in a streamer all follow the same path, and they have a similar composition. That suggests that the stars were born elsewhere, then incorporated into the Milky Way.
One of the biggest mergers happened when the galaxy was just a few billion years old. Astronomers found its remains in observations by Gaia, a European space telescope. The craft found tens of thousands of stars that move in the opposite direction from most of