Question: How are galaxies and stars made?

Sam Geen answered on 17 Jun 2013:

Galaxies are made when dark matter and hydrogen (and helium) gas collapses under gravity. The gas cools down by radiating away energy, which reduces its pressure and allows it to collapse further. A disk forms because the gas usually has some rotation energy that stops it from all collapsing into a single point, and instead forms a disk with spiral arms. Eventually it breaks up into clumps that form stars. Stars begin to shine when the hydrogen gets dense and hot enough to begin atomic fusion. More gas keeps streaming into galaxies from outside in streams known as “filaments”. Often smaller galaxies are swallowed up by bigger ones, which makes them bigger.

So it’s lots of gas collapsing under gravity, falling into spiral disks, and fragmenting into clumps.