https://www.scientificamerican.com/article/the-galactic-collision-that-reshaped-our-milky-way/

Roughly 10 billion years ago the Milky Way—then a smaller galaxy that did not contain its current spiral structure or diffuse halo of surrounding stars—suffered a massive head-on collision that shook it to its very core.

That is when our galaxy’s gravity pulled a smaller companion, roughly one quarter its mass, into a dangerous dance: One where the dwarf galaxy plunged into and out of the Milky Way’s disk, oscillating back and forth until it was finally swallowed whole. Although our galaxy survived, it has never been the same. The collision scrambled the orbits of stars in its disk, making it much puffier, and sent alien stars flying all around the Milky Way, thus building much of its halo. The smash up also funneled new gas toward the galactic center, adding fuel that mixed and mingled with the Milky Way’s existing reservoirs to form new generations of stars.