Here you can create a Universe. Simply put in the parameters you want below and watch the computer get to work!
The amount of dark matter in your universe is key. Dark matter is what allows galaxies to form. Without it, there is not enough gravitational force on the gas to keep it from separating.
This one can be a little tricky. At the centre of galaxies, there is a supermassive black hole. When gas falls onto this hole, it heats up to a very high temperature. This setting decides how much of that hot gas is thrown back out into the galaxy when this happens.
Low mass stars have very long lifetimes. They hang around forever and don't really do anything interesting.
Medium mass stars, like the Sun, have medium lifetimes and make up a reasonable amount of the stellar population.
High mass stars have a very short lifetime. They burn all of their fuel really quickly and then explode in a supernova! Too many of these and your galaxies will blow themselves apart.