From Shelly Midsummer
Angular momentum leads most galaxies to flatten out over time, with all their planets, stars, asteroids and other objects existing roughly on a single plane. While the trajectories of individual objects within a galaxy may change, their collective angular momentum as they spin around their centre of mass remains constant. Picture a plane that is perpendicular to the galaxy’s axis of rotation with objects initially moving above and below the plane. As these objects collide any upward or downward motion is progressively cancelled out, while the conservation of angular momentum dictates that they must keep spinning around the same axis, resulting in a flat, disc-like shape.
Answered by Alexandra Franklin-Cheung for Brain Dump, How It Works issue 107