Can dark matter make dark stars?

Chris Smith put Eva's question to Cambridge University astrophysicist Francesca Day...

Francesca - Dark matter is matter that we know is out there in our galaxy and in the universe because we can see its gravitational impact on other matter, it makes other matter move faster from its gravitational pull, but we can't see it at the moment in any other way. We can't see any light or anything that it emits, as far as we know it doesn't emit any, and we can't detect it in any other kind of lab based experiment on Earth. So most of the matter in the universe we don't understand even though the regular matter we understand really really well. There is this kind of big tension within physics that there is this small group of stuff, the stuff that the Earth is made of, that we really know what we're doing and then most of the stuff out there we've just got no clue.

Chris - Could it though, because it's gravitationally active like the matter we're made of is gravitationally active, could it all clump together and make a huge great dark matter star, which is what the question is all about?

Francesca - Maybe. It depends what exactly dark matter is. In some models of dark matter you do get things called dark stars and by stars what we mean here is an object that has an inward force, so it's a clump of dark matter, there's an inward force set up by gravity and there's an outward force set up by some kind of pressure. So if dark matter interacts with itself, if it interacts with other dark matter, then it could form a star like object, but the star wouldn't shine in the way that our stars do.

Chris - I was going to say, what color would it be?

Richard - Well that was my question actually, is dark matter necessarily dark?

Francesca - It has to be darkish. There are limits on how much it can interact with visible light, but it doesn't have to be completely non interactive with visible light. And indeed we hope it interacts a bit otherwise we’ve really got no chance of discovering what it is.