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If we assume that mass was initially uniformly distributed in an area, then why does it flatten out after long time?

For example, consider our solar system or most galaxies out there.

Udayan Joshi
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    Some confusion here. Your question seems to be about galaxies, but you mention our solar system as an example. The solar system is not a galaxy. Can you edit your question to clarify. – James K Mar 16 '17 at 17:33
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    Minutephysics has a very nice quick video that explains this for solar-systems. https://www.youtube.com/watch?v=tmNXKqeUtJM – userLTK Mar 16 '17 at 18:45
  • Also Why are galaxies disk shaped? deserves a better answer, because it's not enough to say "gas will radiate away some components of angular momentum". – kubanczyk Mar 16 '17 at 20:05
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    I agree with @kubanczyk. Physics.SE has a much better answer: http://physics.stackexchange.com/a/93844/70207 – pela Mar 16 '17 at 20:51

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