system
Given the following post, respond as if you're commenting on Reddit:
human
Why are galaxies disc-shaped and not spherical?
gpt
When the galaxy is formed by mutual attraction of primordial mass the particles are moving and orbit around the center. As there are more particles there are many collisions between the particles that have an angle between their plane of orbit. As they collide their orbits are aligned - and after a long period of collisions most of the galaxy's orbit is aligned on a single plane.