system
Given the following post, respond as if you're commenting on Reddit:
human
Why don't dark matter halos around galaxies collapse to form compact structures like stars and "dark matter galaxies" just like baryonic matter does?
gpt
A good question and very related to why we call it *dark* matter. 

I will however have to start with some basic astrophysics. It is easy to reason that if you put some matter in space it will be pulled together by gravity. Since there is nothing else there it will just all end up in a point. To believe this is a mistake however. You are forgetting about conservation of energy. When matter gets pulled together potential energy is converted into kinetic energy. At the time the matter reaches the centre it will have a large velocity and get thrown out again.

The question to ask here is: How can we get rid of kinetic energy to collapse this structure. For normal matter the key is electro magnetic interactions. The matter being compacted will interact with it self. The ordered kinetic energy will be converted to heat. The heat is then radiated away as photons. The result is that the matter slowly losses energy and can collapse.

Now back to dark matter. Dark in this context mean that it does not interact with light. Since it does not interact with light it has no way to get rid of kinetic energy. This means that the dark matter halos will only collapse to the point where the kinetic energy balances out the potential energy. This size is quite a bit bigger than a typical galaxy.