ELI5- Why are galaxies not shaped as Spheres?
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When galaxies are younger, besides the dark matter, they are mostly gas and plasma. This gas clumps in a center so a lot of mass end up there, perhaps as quasars or primordial supermassive stars. This means there is a lot of gravity in the center.
Then the rest of the gas accelerates toward it but because of the gravity well a lot of it end up in orbit around the center. The bulk of the orbiting matter makes the whole system a rotating body. This means there becomes a desired plane of rotation, as the system now acts like a giant magnet.
When this rotating gas goes on to form stars it already is in orbit around the center so this disc shape hangs around.
Another reason is because any rogue orbits at other angles are more likely to collide, while the ones on the plane of rotation are not. So it results in a lot fewer unconventional orbits than not.
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