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In an effort to find Dark Matter, what did we find? Let's zero in on the matter.

Hosted by: Hank Green
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The great thing about science is that sometimes, some of the smartest people in the world can work on the most sensitive detector of its kind, getting every little piece of it working perfectly - and they can still find nothing.

Because not detecting anything does not make an experiment a failure. It's just a null result, like some of history's most important experiments. And it can still teach us a lot about the universe.

And that is exactly what happened at the world's most sensitive dark matter detector: Last week, astronomers announced that after twenty months of searching, they still hadn't detected a single sign of dark matter. Everything that astronomers can see in the universe is made out of the same kind of matter, the stuff that we're made of - stuff like protons and electrons. That's why they tend to call it "ordinary matter".

But astronomers have known for almost a hundred years that there's more than just ordinary matter out there. By looking at things like how quickly stars orbit the centers of galaxies, and how light gets distorted by gravity, they've realized that about 85% of the mass in the universe is what's known as dark matter.

Dark matter doesn't emit any light - hence the name. So, of course, no one has actually ever seen dark matter, but we know that it's out there because of its gravitational pull. Some scientists think that dark matter is black holes left over from the early universe, which is one of the things that gravitational wave detectors like LIGO are starting to test.

But some of the most popular candidates for dark matter candidates are called WIMPs: Weakly Interacting Massive Particles. Dark matter could be made of lots and lots of light WIMPs, or fewer, but heavier WIMPs. And the Large Underground Xenon - or LUX - dark matter experiment is looking for these WIMPs. LUX is exactly what it sounds like: a huge underground tank of xenon that's designed to detect dark matter. See, WIMPs, if they exist, would interact with ordinary matter through the weak nuclear force, the force responsible for things like radioactive decay. And those interactions can leave traces, if you know where to look.

So LUX used a tank with a third of a metric ton of liquid xenon, with the hope that a WIMP would come and crash into one of the xenon atoms every once in a while. That would make the xenon release a shower of particles that would work their way out of the tank to a collection of incredibly sensitive detectors. Xenon is perfect for this kind of experiment because it's really sensitive to the weak force, but it's not so sensitive that it releases particles on its own, without being hit by a WIMP. And while there are other WIMP detectors out there, LUX is the most sensitive one in the world.

The scientists running the project turned LUX on for twenty months, ending in May of 2016. And after weeding through all their data, they have announced that they did not see evidence of a single WIMP. But that does not mean that there aren't any WIMPs out there.

LUX's findings suggest that WIMPs, if they're out there, must be much rarer than most people thought they were - which means that they're probably the heavier kind. So even though it didn't detect anything, the LUX dark matter experiment excluded a huge range of lighter WIMP models. Null results like these can advance physics and astronomy just as much as the big groundbreaking discoveries that everybody hears and talks about.

LUX is shut down now while it's being upgraded. By 2020, the project's researchers hope to have 30 times as much xenon in the tank. And with more xenon and other improvements, LUX's second run should be at least seventy times more sensitive than the first one was. That'll let it search for a much wider range of WIMPs, including those more massive ones.

So in just a few years, we might know for sure whether dark matter is made of WIMPs. And if it's not, then physicists will know that they should start looking somewhere else. Either way, we'll know a whole lot more about the universe.

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