Dark matter detected in the Milky Way

17 February 2015

Interview with

Dr Chamkaur Ghag, UCL

The subject of this story makes up a significant fraction of the mass in our The Milky WayUniverse. We can't see it, or weigh it - but we know it has to be there. This elusive substance is called dark matter, and now a paper in the journal Nature Physics, has found evidence that it's all around us. UCL's Chamkaur Ghag, who wasn't part of the study but pursues his own research detecting dark matter, here on Earth, explains the significance of the discovery to Kat Arney...

Chamkaur - We can look at galaxies sort of outside of our own and measure, in particular, the velocity of stars on the very edges of those galaxies. And you can see those stars on the edges are  just whizzing around far too fast, if they were being held in just by the gravity of all the other stars in there. And that's telling us that there's this extra mass, there's this extra gravity coming from this dark matter that's all around that galaxy. And that's all well and good, sort of, looking out at other galaxies but it would be quite nice to know what the distribution or how much dark matter there is in our own galaxy. And that's really important if we're going to try and detect it here on Earth. It would be quite nice to know just how much there is, is there any in our galaxy, and just, is there any sort of around our local neighbourhood. And this paper is telling us that yes, there is.

Kat - So, by looking at how stars and things like that in our own galaxy are moving, they can infer that, "Oh, there must be this much push from the dark matter here, there must be this much dark matter here."

Chamkaur - That's right. More of a pull really. They're kind of saying this is all being held together to allow these stars and other gas clouds to be whizzing around at the speeds that they're moving around at. They must be being held on by a lot of sort of force, a lot of strength there, and that must be dark matter. They've got a really strong sort of statistical significance here and stating that there is dark matter here in our own galaxy.

Kat - That's still pretty big. Is there dark matter on Earth? Could there be dark matter here in this office?

Chamkaur - Yes. As the Earth is sort of going around the sun and as the sun is moving around the centre of the galaxy, we'd be feeling, experiencing a sort of dark matter wind coming at us. And so, this dark matter wooshes through the Earth all the time and for every sort of pint of volume, any pint glass you hold up, there's roughly one dark matter particle in there, we think. And it's moving through at about 220 km a second. So, it's whizzing through all the time. There's millions of these things going through your body all the time. It's just a case of, do any of them interact, do they scatter, do they sort of bounce off regular atoms? If they do, great because then we've got a chance to actually find them with terrestrial detectors that we put underground.

Kat - Now we know roughly I guess how much dark matter we think is in our galaxy, what next? What's the significance of this finding?

Chamkaur - What this paper has allowed us to do is really increase our sort of statistics in mapping how much dark matter on average there is in our own Milky Way and extending out to where we are right now. And it's giving us a good handle, some nice evidence, some confidence for when we do go and search for dark matter. It's really telling us that, "Look, it's okay. There is stuff here." We would expect to see this stuff. If it interacts away, we think it does. And if we can start to do this with more and more precision with greater accuracy, we might start to figure out the distribution of dark matter also. We might be sitting in a pocket where there's a lot of dark matter or there's not much dark matter. And that really impacts our chances for finding the stuff.

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