system
Given the following post, respond as if you're commenting on Reddit:
human
Are there complete galaxies of dark matter in our universe? And if so, can they see our matter?
gpt
There *may* be "dark matter galaxies", but they won't have "dark planets", "dark life" etc.

Dark matter makes up most of the mass of matter in the universe, so when gravity causes dark matter to condenses into a galaxy-sized blob, it pulls the normal "baryonic" matter along with it - stuff made of protons, neutrons, and electrons. Baryonic matter can interact with itself in complex ways, and these interactions allow baryonic matter to cool, condense, and form complex structures like planets and monkeys.

However, dark matter does not interact with itself with the same strength and complexity. Reactions involve giving or taking energy, and that energy has to come from or go to somewhere, and some of this energy would come out as electromagnetic energy: light (visible or invisible), radio waves, x-rays etc. That is, if dark matter were capable of forming things like humans and telescopes, it would not be dark matter, it would have to be visible in some way: there is no such thing as "dark light".

This means you can't get galaxies of planets/stars/coffee etc consisting entirely of dark matter.

*However*, while baryonic matter *tends* to pour into blobs of dark matter, this isn't a fundamental law of nature. If there's not enough baryonic matter around, or if the gas and stars have been stripped off some other effect, then it is perhaps indeed possible to have a "dark galaxy" - a big blob of dark matter without a galaxy of stars and gas in it. But this is just an blob, it doesn't contain substructure and can't develop life.