1. Our Galaxy (The Milky Way) used to be a gigantic clump of gas hundreds of light years big.
2. As stars and solar systems started to form it started to sink and make it thinner.
3. Eventually the gravity started to rip it apart (causing the arms of the Milky Way).
4. Present Day: Billions of solar systems have formed and Milky Way starting to settle down.
5. Far Future: Milky Way turns into a elliptical galaxy.