This question already has an answer here:
- Why are some galaxies flat? 4 answers
I'll probably have to delete this question because someone's already asked it, but what accounts for the stable 2-d structure of spiral galaxies in three dimensional space (assuming random starting vectors)? The traditional centrifugal force answer doesn't account for the fact that purely Newtonian $n$-body simulations never evolve into flat disks.
Is it actually suggested that there is some non-Newtonian process to account for this? Or that spinning galaxies throw out stars from the center? I keep hearing a lot of bullshit.
[Edit: I'm keeping this answer for now as it is spurring some interesting discussion.]