Space
Galactic Cannibalism: The Milky Way's History of Mergers
The Milky Way did not grow peacefully. It reached its current size by absorbing smaller galaxies — tearing them apart with tidal forces, stripping their stars into long streams, and incorporating their material into the disk and halo. One major collision 10 billion years ago reshaped the Milky Way's entire inner halo. Another is happening right now. A third, larger one is coming in 4 billion years, and there is nothing that can stop it.