https://www.vice.com/en_us/article/3kx54v/a-nearby-cannibal-galaxy-is-going-to-eat-the-milky-way-scientists-say-andromeda

Andromeda, our closest galactic neighbor, has swallowed multiple smaller galaxies over its 10-billion-year lifespan. Our own galaxy, the Milky Way, is destined to be Andromeda’s next and most calorific meal, once the two galaxies collide in about four billion years.

To better understand Andromeda’s past diet—and to anticipate what might be in store once the Milky Way merges with it—scientists studied the complex gas halo surrounding the galaxy in unprecedented detail in a new study published on Wednesday in Nature.