Eli5 Why are galaxies flat instead of being spherical like stars and planets?
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The best answer I can give is that once an imbalance starts with something going around, it starts to flatten in a manner that mimics that. The winning pizza dough idea doesn’t really work because something outside the dough introduced the spin. Explaining the geometric formation of galaxies is more akin to pizza dough becoming a round disc when left to its own devices.
If you have a flat galaxy and something just appears above it, all of the mass is below it and pulling it down. It will fall through and pop out of the other side, but in doing so it will likely get close to other things that will help pull it in behind them. They dance around and something else pulls them down flatter, then something else, then something else, etc.
Just imagine that as it falls down, the way it falls in line is similar to a ripple throughout everything orbiting. Not so graceful at first but it’ll mellow out over time.
Now rewind the idea back as far as you can and you can see how it forms. It just takes a tiny bit of a random imbalance with angular velocity.
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