‘One of the biggest discoveries in science’: have scientists found dark matter?

0:00This is The Guardian.

0:13It's one of the biggest mysteries in science, an elusive substance which has been hunted by physicists for decades, and now a tantalising piece of evidence. Maybe. A finding that could unlock a new understanding of our universe. So today, have scientists finally caught a glimpse of dark matter? From The Guardian, I'm Madeleine Finlay and this is Science Weekly.

0:54Ian, we're here to talk about a potentially very exciting finding for physics about dark matter. But before we get on to the finding itself, let's talk about the background. What do we need to know about dark matter?

1:09Madeleine Finlay: Dark matter is this mysterious, invisible substance that seems to clump around galaxies. Now, it doesn't emit, absorb or reflect light or any other electromagnetic radiation. That makes it really difficult to see. It's invisible, but it makes it also very hard to detect. And the way we really know about dark matter is through observations of the sort of gravitational effects it has on other things. And what we can tell is that dark matter seems to form this, it's often called a cosmic web. It sort of seems to form a kind of structure for the universe, okay? Now, two people who are really key in getting some understanding of dark matter out there.

1:54Madeleine Finlay: The first is a guy called Fritz Zwicky. He was a Swiss-born astronomer. And in 1933, he started looking at clusters of galaxies. And he noticed that there wasn't enough visible matter in these clusters of galaxies to prevent the galaxies from just flying apart. and he said, oh, maybe there's this stuff called, he called it dunkler materia. That didn't have a massive impact at the time. But about 40 years later in the 70s, a US astronomer called Vera Rubin, she looked at individual galaxies. So she zoomed in on individual galaxies and looked at the speeds of the stars, especially around the periphery of those galaxies.

2:33Madeleine Finlay: And she worked out that the speeds of those stars at the edges of those galaxies just didn't make sense unless there was some other matter there that wasn't visible, but there was exerting a gravitational force on those particles. And so now the sort of thinking is that galaxies are forming and exist within these kind of clouds of dark matter. So you could say that right now dark matter is a set of observations with a sort of enigma at their heart. But why is figuring this out such a holy grail for physicists and people who study the universe? Dark matter is thought to make up about 85 % of the matter of the universe.

3:21Madeleine Finlay: That would leave what we call normal matter, the stuff of you and me, making up about 15%. It's extraordinary that we don't know what makes up the vast majority of the matter of the universe. That is staggering. And so to understand this is an enormous question in physics. It's all clearly a bunch of particles that are unobservable to us, at least directly, but we don't know what's there. And that is super intriguing. So you said they're unobservable, they're non-interacting. Presumably this is why it's been so hard to find out what exactly dark matter is. Yeah, so not only is dark matter invisible, it also sort of barely interacts with other matter.

4:09Madeleine Finlay: So it streams directly through you and me, the stuff of normal matter. And so it's really hard to detect. The other thing that makes it tough to find is that if you're looking in a detector for dark matter, you know, you're looking for incredibly rare collisions or interactions between particles. The thing is, particles are interacting all the time. And so you have these huge physicists call them backgrounds. You have background noise, essentially, from other particles interacting, radioactive decays, particles coming in from the solar wind, from space, all sorts of particles constantly crashing into each other, producing not only background noise, but sometimes also signals that can be mistaken for signals for dark matter.

4:55Madeleine Finlay: So it's an incredibly hard thing to find.

5:13OK, Ian, so this brings us on to this latest finding, if I can use the word finding. Tell me about this experiment.

5:21Madeleine Finlay: This result is from what's called the Lux Zeppelin detector, the LZ detector, and it's one of the world's most sensitive dark matter detectors at the moment. It's built in an old gold mine in South Dakota. It's nearly a mile underground and that's to shield it from a lot of the particle interactions, a lot of the noise that we were talking about earlier. And at the heart of the detector is 10 tons of ultra-pure liquid xenon. Xenon's a noble gas. You can make it liquefy it and it becomes an incredibly dense liquid that you can use as a detector. Now, the aim of the detector is that a dark matter particle might get far enough to this detector, into this fluid, and strike a xenon atom, actually interact with it and produce a flash of light.

6:12Madeleine Finlay: Now, there are photosensors all the way around this tank which will detect that light. And by analysing the light, you can try and infer some stuff about the particle. What happened in that interaction could tell you something about, for example, the mass of the particle involved in hitting the xenon atom. Right. So what have these researchers seen? What the researchers reported was a single event that might, and I have to repeat it might, have been triggered by a postulated dark matter particle called a weakly interacting massive particle, the beautiful acronym of a WIMP. OK, and if they're right, this particle would actually be a pretty heavy version of a wimp.

6:56Madeleine Finlay: It would be about 200 times the mass of a proton, proton being the sort of positive subatomic particle inside an atomic nucleus. And they say they have only one event, but they are reasonably excited about it. And I wanted to find out more about this. So I called up Dr. Samuel Erickson. He's a senior research associate at the University of Bristol. And he's also the first author of the research paper that the LZ detector experiment put out discussing this new finding. In this latest analysis that we've just announced, we're looking at data we collected over about a year from 2023 to 2024. And the exciting thing is we have this one event which doesn't look like any of the backgrounds that we know about.

7:45and so it could be some super rare background that no one's ever seen before and no one fully understands or it could be the first hint of dark matter but i emphasize it's just one event so we shouldn't get too excited but it's such an outlier that it's exciting nonetheless so as sam

8:07Madeleine Finlay: says this is really exciting so i wanted to know what it felt like when they saw in their data what might be this interesting potential first glimpse of dark matter the way we do uh our bias mitigation is through this process called salting so we inject fake events into our data stream and so we never really know whether what we're seeing is a fake event or a true signal event and so i started this analysis just over two years ago and so i've been staring in this event for just for about two years always thinking oh maybe it will disappear as we you know understand our data a bit more but it's stubbornly remained and then when we unsalted so took out all of these fake events that happened about two months ago that was actually when the excitement really really built so like oh we've got this event we cannot explain it at all we got we got to tell people.

9:14I can imagine seeing that in the data must have been very exciting. But how far away are we from calling this a detection of dark matter?

9:28Madeleine Finlay: I mean, let's be clear that nobody on the experiment is claiming to have found dark matter. What they're saying is we've looked at everything else we think it could be and we can't explain it so we're going to put it out there we're going to let people discuss it and we you know that's all we're doing the good thing about particle physicists is that they have pretty strict sort of thresholds statistical thresholds that you have to meet before you can claim a discovery so this is all kind of formalized what you want to do and we saw this with the discovery way back of the the higgs boson was that you need the statistical significance of your discovery to reach what's called five sigma before you can officially claim a discovery okay now five sigma is equivalent to a one in three and a half million chance of it being just a fluke so very low okay the result that they've got the result that the lc detector have got at the moment is about 2.6 sigma and that is more like a one in 200 chance of it being a fluke.

10:35Madeleine Finlay: So 1 in 200, you know, sounds, well, you know, it still feels pretty certain. But the thing about particle physics is, with these kinds of experiments, you do so many experiments, there are so many events going on that you have to go through, that 1 in 100, 1 in 200, these kinds of signals can come and go. So you don't, you try not to get too excited about them until you hit that five sigma level, because it's just too much of a noisy environment.

11:07Coming up, if this is dark matter, what happens next?

11:21Madeleine Finlay: He looked at me and he said, if something ever happens to me, release the AI. Your husband goes down an AI rabbit hole. How do you pull him out? I just kept asking myself, what is happening? Black Box, The Chatbots, a new series from The Guardian Investigates about the strange things happening between AI and us. The whole series is out right now. Search for Black Box wherever you get your podcasts.

11:52So Ian, what's the reaction been from the wider physics community?

11:58Madeleine Finlay: I really wanted to know this myself. And as we know, physicists, they really like to cross their T's and dot their I's before they put anything out there. And so this was something I asked Sam about, like, you know, what is the mood among you lot right now? So the mood is, I'd say, remarkably positive. So we put the paper out that describes all of this as a pre-release last week. And there's already been a dozen or so follow-up papers trying to explain it with various theoretical models. So that's always quite exciting to see. But it's also, you know, dark matter has been searched for for 100 years, for coming on 40 years with this kind of technology.

12:45This is the first observation of this kind. So it's very exciting in that regard. And my inbox is largely filled with questions of when are you going to give us the results for the rest of the data? So as we heard Sam say there, people are clamouring for the next stage of results. What will that cover? What happens now?

13:09Madeleine Finlay: The simple answer is that the scientists just need more data. They need to gather more data. They need to analyse more data. Now, this event that we're talking about struck that detector, was recorded in that detector on the 16th of June, 2023. And they've analysed data up till April 2024. So they already have nearly two and a half years more data than they've analysed. Now, obviously, they're now keen to analyse that extra data to see if there are any more events in there. And if they find more events of the same mass, they will start adding to this and building confidence and they will be on their way to a discovery.

13:50Madeleine Finlay: But there are also other detectors. There are other dark matter detectors around the world that people are using and they will clearly be looking at this result and being very interested and thinking, can we look in our data? Can we tweak our machine to be sensitive to this particular mass range of particle? And there are plans for future detectors. One of them could be a detector that would be about four times more sensitive than the LZ detector. And it could potentially even be put into what's called Bulby Mine, which is an old mine in North Yorkshire, which would be obviously absolutely fabulous for the UK.

14:25If this does turn out to be the first evidence of dark matter finding the missing 85 % of the universe, I mean, that's big, big, no?

14:40Madeleine Finlay: This would be one of the biggest discoveries in science, because we're talking about understanding what is the vast majority of the mass of the universe. I mean, this would be vast. Clearly, it would be Nobel time. But it would also just be opening up this massive new realm that until now is completely hidden from us. What if it's nothing? What if it is just statistical randomness? You know, this result could disappear. We could be not talking about it in a couple of years' time. It may just fade away and the search goes on, right? You know, these signals do go away and we can't get our hopes up just yet.

15:29No, but I know there will be plenty of scientists out there keeping all their fingers crossed. You know, they want to see this solved in their lifetimes.

15:38Madeleine Finlay: Yeah, and you know, that could well happen, even if this result doesn't pan out right. That's true. Ian, thank you so much. Thanks again to Ian and to Dr Sam Erickson. And before you go, I want to recommend a series all about another mysterious phenomenon we don't really understand. AI. The Guardian's chart-topping podcast, Black Box, hosted by Michael Safi, is back for a second series. and the new season is all about our relationship with AI chatbots and how they're quietly reshaping lives. All six episodes are out now, so search for Black Box The Chatbots wherever you get your podcasts. And that's all from us.

16:25This episode was produced by Josham Chana, the sound design was by Joel Cox and the executive producer was Ellie Burie. We'll be back on Tuesday. See you then.

16:42This is The Guardian.