Disc of Galaxy

Sombrero spiral galaxy from NASA and the Hubble Heritage Team (STScI/AURA)

In young spiral galaxies like the Milky Way, not all the gas was pulled into globular star clusters. Some of it was left behind, and gravity pulled this in a different direction. It fell down towards the middle of the galaxy. The galaxy was slowly spinning round, and this spin threw the gas out into the shape of a flat disc with a bulging centre. We do not know when the disc formed. We guess it was around 12 billion years ago.

Thickness of disc of Galaxy

In the PennySystem model, in which the Solar System is the size of a penny, the disc of the Milky Way Galaxy in which we live would be 300 meters (about 1000 feet) thick.