Because we have no dark matter planet in our solar system, can we assume that dark matter is not sticking to itself?

We put Donald's question to Andrew Pontzen... Andrew - Well very quickly the answer is yes. Dark matter is this extra stuff that we seem to need to make sense of the universe when we look at the way that galaxies, which are collections of hundreds of billions of stars. When we look at the way that they behave and, in particular, the way that they're spinning around and things are moving within them, we can see quite clearly that they're spinning too fast to be consistent with their only being made out of the stars we can actually see. And what we think is going on is the gravity that actually causes gravity to spin is being given an extra boost, if you like, by having some extra stuff there that we can't see directly. And that's the stuff we call dark matter.

At that level, of course, it's a bit made up. It's a sort of made up solution to a problem. But then we run with that idea and come up with a whole load of predictions for other things in the universe which then turn out to be correct. So this idea of dark matter does seem to be onto something. On the other hand, we've never been able to really figure out what it is. Nobody has found a bit of dark matter in a laboratory. That's probably to be expected because it seems to be invisible, but that means that it's properties - is it sticky for instance? It's stuff that we don't really know the answer to and we need to start thinking about exactly those kind of questions.

Chris - Kerstin.

Kerstin - What do you think, will we soon know, are we going to find dark matter at any point?

Andrew - Well, that is a very big question. I think eventually we will find it. But I think right now the prospects for finding it in the near future might not be as great as some people would like to think. And so we do need to look out into the universe to figure out what are its properties, what should we be really looking for? So this kind of argument where you say there aren't planets made out of dark matter therefore it can't be sticky is a godsend, it really is, because we can start to understand what is making dark matter tick.