TED Ed: Why is the Milky Way a Spiral?

"Explore the ever-evolving structure of the Milky Way galaxy, and find out how it got its spiral shape."

Thirteen billion years ago, the gas and dust particles that eventually became our Milky Way were whizzing around in every direction. Despite the wild motion of its parts, the galaxy as a whole was rotating about an axis. So why did the Milky Way, like most galaxies, become flat, and not spherical like stars and planets?