Hunting for WIMPs: What Might Dark Matter Actually Be?

Scientists have built detectors buried more than a mile underground in old mines, just to try to catch a single tiny particle they have never actually seen. That particle might be dark matter — an invisible substance that makes up about 27% of everything in the universe, far more than the roughly 5% that is regular matter, the stuff that makes up stars, planets, and you.

By the end of this lesson, you will be able to: - Explain why scientists believe dark matter exists even though no one has seen it - Name at least two leading candidates for what dark matter might be - Describe how one real detector tries to catch dark matter - Explain the difference between dark matter and dark energy

In the 1970s, astronomer Vera Rubin studied how fast stars orbit around the edges of galaxies. Based on the visible matter in those galaxies, the outer stars should have been moving slowly, the same way outer planets in our solar system orbit the Sun more slowly than inner planets. Instead, Rubin found the outer stars were moving just as fast as the inner ones. Something invisible had to be adding extra gravity to hold those fast-moving outer stars in place. Scientists named that invisible something dark matter.

Scientists have proposed several candidates for what dark matter could be made of: - WIMPs (Weakly Interacting Massive Particles): heavy particles that almost never interact with normal matter, which is why they are so hard to detect - Axions: particles predicted to be extremely light, first proposed to solve an unrelated puzzle in particle physics - MACHOs (Massive Compact Halo Objects): things like dead stars or black holes that are dark simply because they don't give off light — mostly ruled out by observation, since there are not nearly enough of them to explain all the missing gravity - Primordial black holes: black holes that may have formed in the very early universe, still being studied as a partial explanation

One real experiment, called LUX-ZEPLIN (LZ), sits nearly a mile underground in a former gold mine in South Dakota. It uses a tank holding about 10 tons of liquid xenon. The idea: if a WIMP passes through Earth and happens to bump into a xenon atom, it should produce a tiny, specific flash of light and charge that sensitive detectors can record. The lab is buried so deep specifically to block out cosmic rays and other particles from space that would otherwise flood the detector with false signals.