SCOTT DETROW, HOST:
Dark matter is one of the strangest puzzles of modern science. This invisible substance makes up about 85% of the total matter in the universe - much, much more than the stuff we can see. Yet despite dark matter's abundance, scientists still have not directly detected it after decades of searching. One of the experiments built to detect it sits in an old gold mine about a mile underground in South Dakota. And scientists there just announced a strange event they cannot explain, renewing excitement around the hunt for dark matter. Here to unpack it all with us is Priyamvada Natarajan, chair of the Department of Astronomy at Yale University. She's a dark matter expert but wasn't part of the experiment. Welcome.
PRIYAMVADA NATARAJAN: Thank you. Delighted to be here.
DETROW: What is it like to be an expert in something nobody has ever verified or seen?
(LAUGHTER)
NATARAJAN: Well, you know, all my work is relevant to the invisible universe. So you can ask...
DETROW: Yeah.
NATARAJAN: ...What does one do with a lifetime dedicated to the things that are not seen? Well, they're not directly seen, right? So that's what's amazing about dark matter. Even though dark matter itself does not emit, absorb or reflect light, right? Which is how we actually - light is the cosmic messenger. And because it doesn't interact with light, we don't actually directly see it, but we see it indirectly.
Dark matter, for example, because it's matter, it has gravity, and so it impacts the motions of objects - right? - of stars that are visible matter, and that's how we infer its presence. And the challenge has always been - like we see this on astronomical scales, right? It's smeared lightly everywhere in the universe but lumps in many places, like around galaxies.
DETROW: Yeah.
NATARAJAN: But making the connection to what particle it might be - what subatomic particle it might be, that has been the challenge.
DETROW: So you were not directly part of this experiment, but I have to imagine you were paying very close attention. Can you briefly explain what they saw in this underground dark matter detector?
NATARAJAN: Yeah. The experiment itself is quite remarkable, right? It's the LUX–ZEPLIN. It's one of the world's most sensitive dark matter detectors. And basically, it's a vat of xenon. So xenon is one of those inert gases that can also exist in liquid form. And one particle - something - seems to have struck a xenon nucleus. And the operative word is one, right? So it's just one event, but it hit it unusually hard. And so they cannot explain this with radioactive particles, neutrons, neutrinos, you know, all the other kinds of effects, you know, detector effects, etc. So they found this event, but it's not significant statistically at the level that scientists feel convinced that it's actually a discovery. It's not a discovery.
DETROW: Why is it so intriguing, though? Like, what is it about something struck something in here that has everybody so curious?
NATARAJAN: The thing is it's completely unexpected - this kind of interaction, right? So we have a lot of models. There's a landscape of models of what all dark matter could be, right? You know, and we have a zoo of particle candidates, and we have made predictions for what - if at all, and ever - we would see them, how they would interact. And so this was a real thwack. So the simplest dark matter models - right? - they predict a lot of gentle collisions and fewer real hard, you know, high-energy thwacks. But here, we didn't find any gentle collisions. We just found one thwack.
DETROW: OK.
NATARAJAN: So that's really, really bizarre because that tells you that this is a very special kind of particle.
DETROW: So what happens next?
NATARAJAN: So I guess we all get busy - both experimentalists and theorists. The experimentalists actually have a lot more data that they haven't analyzed yet. So this event was detected about two years ago, so they have that data on hand, and they're going to continue running the experiment. And all of us model builders and those interested in theoretical ideas, are going to get really busy coming up with new ideas.
DETROW: Well let me ask you to theorize. Hypothetically, if we were to crack the code and figure out what dark matter is made of - which I know is like 80 steps down the line from what we're talking about right now - how would that change our understanding, as best as we can guess, of how the universe is constructed?
NATARAJAN: I think it would be utterly transformative. It would be the most radical discovery of not just this century. Because dark matter dominates the matter inventory of our universe, right? And as you mentioned - right? - only about 85% of all the matter in the universe is dark matter, and the stuff that we are made of is 16% - everything in the periodic table, right? We're like - you know, we're a bit player in the matter game, right? So this is what everything else in the universe is. This is like suddenly knowing what the universe is really made of, right? And, I mean, it really would give us a much better grounding on our place in the universe - right? - like how we really came to be.
DETROW: Priyamvada Natarajan of Yale University, thanks so much.
NATARAJAN: Thank you so much, Scott.
(SOUNDBITE OF CHANTAY SAVAGE SONG, "I WILL SURVIVE") Transcript provided by NPR, Copyright NPR.
NPR transcripts are created on a rush deadline by an NPR contractor. This text may not be in its final form and may be updated or revised in the future. Accuracy and availability may vary. The authoritative record of NPR’s programming is the audio record.
