What we know about dark matter

Why do we think dark matter might exist in the first place? To take us through the timeline of discoveries is Durham University’s Francesca Chadha-Day...

Francesca - The first hints at dark matter were really discovered by Zwicky in the 1930s, and he was looking at galaxy clusters. So these are massive structures. And at that time when Zwicky was working, most physicists assumed that the matter we can actually detect directly with our telescopes was all that was. And Zwicky observed the Coma Galaxy cluster, and he found that the matter, he could see the luminous matter moving much faster than expected. And for matter to be moving that fast without flying off into space, there needs to be a certain amount of mass pulling it in with its gravitational attraction. So he estimated the total mass of the cluster, and he found that that estimated mass was much bigger than could be accounted for by the luminous matter alone, the matter he could see. So this was really the first hint that there was some additional matter in that galaxy cluster, which Zwicky termed dark matter.

Will - How was that theory taken a step further?

Francesca - So the next big piece of evidence came from another astronomer, Vera Rubin. And she was looking not at galaxy clusters, but at galaxies, she saw exactly the same thing. She made detailed measurements of how the velocities of the stars changed with their distance from the centre of the galaxy. And again, she found that the stars' velocities were much too high to be bound to the galaxy by the mass of the luminous matter alone in that galaxy. So again, to make those velocities of stars make sense and to make them not just fly off into space, there needed to be a large additional component of mass in the galaxy, which she concluded would be provided by this dark matter.

As Francesca outlined, dark matter could well be what’s preventing galaxies from spinning themselves loose by nature of just being there and adding some mass to the carousel. But what is the nature of the matter itself? Well, just over 10 years ago, we put in a call to the University of Edinburgh’s Catherine Heymans. She talked us through what the understanding of dark matter was at the time. So, after a decade, are we any closer to answering the question ‘what is dark matter?’...

Catherine - Okay, so there are various different contenders for the dark matter particle. The top contender for a dark matter particle is something called a WIMP, which stands for weakly interacting matter particles, because astronomers just love their acronyms <laugh>. So the WIMP particle is something that interacts through the weak force, and the top contenders for WIMPs are super symmetric particles. So, your listeners might be familiar with this idea of string theory, this sort of underlying theory that can explain our universe. And this proposes that there will be additional particles that are sort of mirror images of the particles that we know and love, the quarks, the bosons, the leptons. So there's that group of particles that could be dark matter. There's also something called axions. Axions are actually super interesting because they come in all different ranges of energy and masses. It's a theoretical prediction, but in principle, the landscape of different axioms that you could have could in principle explain both dark matter and dark energy. So lots of excitement about axions as well.

Will - Well, now it is time to put you on the spot. It's time for our 10 year retrospective on dark matter. 10 years ago on this program, you said that 'we know that it is weakly interacting, we know that it is cold, and we know that nobody has detected the particle yet.' Are we still three for three in that regard?

Catherine - So I would say we are definitely sure it's still cold. So dark matter dictates when and where the galaxies form in our universe. And if you look at how galaxies are distributed across the universe, that only makes sense if there is dark matter around to sort of seed that galaxy formation. And you can look at the distribution of the galaxies and that can tell you about the properties of the dark matter and it has to be cold, which means sort of slow moving. If it was much warmer, then it would have a lot more momentum in the early universe, which would change the way we see the distribution of galaxies today. So I'm still happy with the cold. Now, the weakly interacting, that's still the best contender for dark matter. But what's changed over the last 10 years is that we haven't had a big discovery at the Large Hadron Collider. So the large Hadron Collider at CERN was built to find the Higgs Boson. Success, Nobel Prize, Well done everyone. But it wasn't supposed to find the Higgs-Boson, that was supposed to be the tip of the iceberg. So your models of string theory that predict these super symmetric particles, the lightest super symmetric particle in this theory should have been detected by the Large Hadron Collider by now. That was supposed to be the next thing found. And the fact that it hasn't been found has thrown into question this whole string theory and super symmetric landscape. Now, it could be that they're just much lower mass than low energy that can be detected by CERN. And maybe when it has its upgrade, these particles will pop out. But I think because the simplest theory hasn't been born out in experiments over the last decade, people have been getting more interested in this axion alternative.

Will - And have we been able to detect the particle yet?

Catherine - Nothing yet <laugh>. But the error bars are going down. So you can either create one of these particles or you can catch one of these particles. And the fact that we haven't managed to create or catch a particle yet means you are reducing the limits on how massive or how energetic it can be. And so we're sort of narrowing down the space, but no, not detected yet.

Will - When compared to other dark entities that we will talk about a little bit later on in the show, it feels like we have an idea of where it is, what it's doing and what it could be. It feels like there's a tangible, genuine chance that we might be able to crack it. Do you think we may uncover its secret, say, within our lifetime?

Catherine - If it's a dark matter particle, then yes, I think we will just because the techniques and instrumentation are growing so fast now. We've got the upgrade coming at CERN and also the direct detection community are clubbing together to essentially put all of the liquid xenon in the world in one place. So a really massive butterfly net to catch these tiny dark matter particles. And excitingly, I don't know if you've heard about this, there are plans to install this at Boulby mine in Yorkshire. Can you imagine the headlines, 'dark matter detected in Yorkshire?' I just, I I love the idea and so there is work on the way to develop this mine as a future dark matter detection site, which would be fantastic. So I am hopeful that if the answer to dark matter is that there is a particle, then I would hope that we would detect it in my academic career. But it could be that it's something else entirely. I mean, it's such an open question at the moment, I think. And there's lots of work in different areas to try and explain this phenomenon because the simplest answers haven't panned out yet.