Is dark matter really there and how do we know?

Chris Smith put this question to physicist Francesca Day, who definitely didn't keep us in the dark...

Fran - First I should quickly explain what dark matter is in case anyone doesn’t know. Dark matter is matter that we think is there because we can see the effects of it’s gravity on other matter like stars and galaxies in space, but we haven’t detected it any other way than its gravitational effects. So then you might think well, maybe it’s not that there’s extra matter, maybe we just haven’t quite understood gravity. And, indeed, there are physicists that work on modified gravity as an alternative dark matter.

Chris - What’s modified gravity?

Fran - It’s where you take Newton’s laws of gravity or even Einstein’s theory of gravity and you tweak it to try to fit the observations better.

Chris - That’s called fiddling, isn’t it? Isn’t that called adding a fiddle factor to make the observation fit the fact?

Fran - Yes, absolutely.

Chris - It sounds iffy to me.

Fran - Dark matter is much much more popular because based on just having one more particle in our model of particle physics, and having there be some of these particles around we can explain all the observations. But there are observations that dark matter can explain very easily but modified gravity can’t; for example, dark matter is required to explain how structures like galaxies form in the universe in the first place. You need the gravity of the dark matter to be the seed that hold the galaxies together, whereas modified gravity can’t explain this. So dark matter really explains a lot of different astrophysical observations with only one addition, which is what makes it a very appealing theory but, of course until we detect it we can’t know for sure.

Chris - Why would all the dark matter stick together and be in a big halo round a galaxy for example? Because if it’s gravitationally active, why isn’t it all sort of mixed up with the other material we have, the stars and the planets and so on?

Fran - It is, but it’s more the case that the stars and the planets are mixed up with the dark matter. We think that dark matter probably only interacts significantly via gravity and it’s other interactions are very weak, so the dark matter falls in on itself because it’s gravitationally attracted to the other dark matter and forms these big blobs of dark matter.

Chris - So dark matter is attracted by gravity to other dark matter but it doesn’t interact with dark matter in any special way apart from via gravity we think?

Fran - We think. It might interact with other dark matter using some other forces that we don’t know about yet, but there’s no direct evidence for that.

Chris - And we think it was there what as a product of the big bang? Has it been around since the birth of the universe?

Fran - Yes. In the same way that all the regular matter that we see every day was produced in the big bang, dark matter would also be produced in the big bang at the same time.

Chris - So when scientists talk about there being particles of dark matter whizzing through us all the time, one physicists at UCL in London put it to me that if I had a pint glass, I’d have at least a couple of dark matter particles in there right now. Why is that there? Why isn’t it out aggregated around with the dark matter in a halo then? Is that just a reflection on how much dark matter there is all over the place, all the time?

Fran - Yeah. It’s best to think of the Earth as being in a dark matter halo. The Earth is situated in the Milky Way galaxy, and the Milky Way galaxy is situated in a big sphere of dark matter. So because we’re in this big sphere of dark matter there are dark matter particles around everywhere.

Chris - Will the dark matter go where the matter goes? Is that why we’ve got our galaxy, there’s a mixture of matter and dark matter in the same place in space and time?

Fran - Yeah. They’re gravitationally attracted to each other. But it’s better to think of it as the matter going where the dark matter is.

Chris - Philipe?

Philipe - Because dark matter is then a form of matter, would there be something as an anti-dark matter particle?

Fran - That’s a very interesting question. In some theories of dark matter there is anti-dark matter particles but in some theories there aren't. In the same way that an electron has an antiparticle which we the positron, but a photon is its own antiparticle, it doesn’t have a different antiparticle. It’s the same for dark matter, so some dark matter wouldn’t have an antiparticle but some could.