WIMPs: The Invisible Particles Scientists Are Hunting Underground

What if most of the universe were invisible — and we had no idea what it was made of? That is actually true! Scientists have figured out that about 27% of everything in the universe is made of something called dark matter. Regular stuff — you, your desk, the Earth, the Sun, all the stars — only makes up about 5% of the universe. Dark matter is invisible. It does not glow, it does not reflect light, and we cannot see it with any telescope. But we know it is there. How? Because of the way galaxies spin.

By the end of this lesson you will be able to: - Explain what dark matter is and how scientists know it exists - Describe WIMPs and why scientists think they might be dark matter - Explain why WIMP detectors are built deep underground - Compare what WIMPs are to what they are definitely not

In the 1970s, an astronomer named Vera Rubin noticed something strange. When she measured how fast stars move around the edges of galaxies, they were moving way too fast. Normally, stars far from the center of a galaxy should move slowly — just like the outer planets in our solar system orbit more slowly than the inner ones. But Vera's measurements showed the outer stars were moving at almost the same speed as the inner ones. The only explanation was that a huge invisible cloud of extra matter must be wrapped around every galaxy, adding extra gravity to keep those fast-moving stars from flying away into space. This invisible stuff is what we call dark matter. Vera Rubin found this same pattern in galaxy after galaxy, and scientists have confirmed it many times since then using different methods.

Scientists have many ideas for what dark matter might be made of. One of the most popular ideas is called WIMPs — which stands for Weakly Interacting Massive Particles. WIMPs would be massive (much heavier than a single proton — the tiny particle at the center of every atom), weakly interacting (they almost never bump into regular matter or react with it), and completely invisible (they do not give off, absorb, or bounce back any light at all). Imagine a ghost that could pass right through walls, tables, and even your body without you ever feeling it. That is what a WIMP would be like. Scientists calculate that trillions of WIMPs might be passing through your body every single second — and you would never know. Why do scientists think WIMPs are a good idea? Because the math works out really well. If WIMPs were made in the right amounts during the Big Bang (the beginning of the universe, 13.8 billion years ago), there would be exactly enough of them to explain all the dark matter we see affecting galaxies today. Scientists call this lucky match the 'WIMP miracle.'

Finding a WIMP is incredibly hard. Scientists need a detector that can notice when a single WIMP bumps into an atom inside the detector. But here is the problem: every second, Earth is bombarded by cosmic rays — energetic particles flying in from outer space. These cosmic rays would set off false alarms in any surface detector, making it nearly impossible to identify a real WIMP signal. The solution: go underground. Deep rock blocks cosmic rays, but WIMPs — because they pass through almost everything — still reach the detector. The LUX-ZEPLIN (LZ) experiment, located in the Sanford Underground Research Facility in South Dakota, sits 1,478 meters (nearly a mile!) below the surface. Its detector holds 10 tonnes of liquid xenon cooled to -108 degrees Celsius. If a WIMP ever hits a xenon atom, it would produce a tiny flash of light that the detector's sensors would record. As of 2024, no confirmed WIMP has been detected — but each experiment rules out certain types of WIMPs, narrowing down what dark matter could be. The search continues.