system
Given the following post, respond as if you're commenting on Reddit:
human
Why galaxies are flat? Why there are no spherical galaxies but only disc shaped galaxies?

Gravity should be same in all 3 dimensions then why galaxys are flat , and we  don't see a sphere with a black hole at the centre and stars revolving around it around the whole sphere, why disc shape?
gpt
There are spherical galaxies! We call them "ellipticals" though - they have a bit of angular momentum, so they tend to be somewhat "smushed" or "stretched" spheres.

There is nothing stopping stars from orbitting all over the place - and when disc galaxies merge, often the stars get so stirred up that it forms an elliptical galaxy in the end. However, stars tend to *form* in a disc. We only have disc galaxies because the stars are basically staying in the configuration they started with.

While stars can orbit any way, *gas* tends to form a disc. Stars almost never collide with other stars (except if they form together as a binary etc), but gas particles in space do actually collide with each other quite often. This means that you can't just orbit anywhere you want - the gas particles will bump into each other and lose energy. Eventually it settles down into the lowest energy configuration you can get while still conserving angular momentum - and that's a disc. The stars then form from the gas of this disc, and tend to stay roughly in a disc.

Over time, the stellar disc does stir itself up a bit, and ends up a bit thicker than the gas disc. Generally, we have a "thick disc" of older stars, a "thin disc" of younger stars, and a very thin disc of gas.

As a side note: the supermassive black hole isn't really important here, as far as gravity goes. The vast majority of the mass is in dark matter, stars, and gas. Stars orbit within their own shared gravitational potential - they aren't affected by the supermassive black hole unless they're basically right on top of it.