Why did the Milky Way form a disk instead of a perfect sphere?

Early in its history, the Milky Way gathered gas and dark matter within a growing halo. As the gas cooled, it collapsed toward the plane of rotation, and conservation of angular momentum caused it to flatten into a disk. Stars formed in that disk inherited the same general motion. The result is the thin, rotating disk we see today, along with a thicker stellar halo and central bulge. Our galaxy’s structure is therefore a record of both collapse and long-term growth through mergers.