system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are galaxies more or less "flat", rather than spherical?
gpt
Because they spin.

When you have loosely connected matter, like the dust from which galaxies are formed--or, say, pizza dough--as it rotates, it tends to push material away from the axis of rotation. Thus, gravity can compress the matter into a disc-like shape, but the faster it rotates, the harder it is for it to compress into a spherical shape.

This applies to the formation of many objects in astrophysics--it's why solar systems tend to have a 'plane' much like galaxies, and even stars are originally formed from a collapsing disk.

It's also worth noting that there *are* more spherical galaxies, as well as a large central bulge in otherwise 'flat' galaxies. There's quite a lot of variance.