Researchers at the XENON dark matter observatory have spotted something incredibly rare. Unfortunately, it’s not dark matter, but it is the next best thing. The detectors at the observatory have spotted the decay of xenon-124, the rarest event ever recorded in human history.

The XENON experiment is designed to detect dark matter, which is not an easy task. The reason that dark matter is so mysterious is that it pretty much never does anything, which makes it hard to spot. Dark matter doesn’t give off light, or have any sort of magnetic field, and it almost never interacts with normal matter in any way.

To even have a chance at seeing the effect of a dark matter particle, scientists have had to come up with a bunch of outlandish experiments. One of those is the XENON observatory, which is essentially a giant tank containing 3.5 tons of liquid xenon sitting in a laboratory nearly a mile underground.

The experiment uses xenon because it’s one of the most stable elements in the universe. Pretty much nothing reacts with xenon, especially a mile underground, making it the ideal dark matter detector. After all, dark matter reacts with xenon as little as it reacts with anything else, so the XENON team can crank the sensitivity on their equipment to 11.

While that hasn’t been enough to find dark matter just yet, it was enough to spot the decay of a xenon-124 atom. Xenon-124 is a radioactive isotope that takes about 1.8 sextillion years to decay, on average. That’s about 130 billion times longer than the lifespan of the entire universe until now. Seeing a xenon-124 atom decay is, shall we say, uncommon.

And yet, the XENON observatory spotted one, and the detector was sensitive enough to pick it out and record it. If the XENON detector can spot an ultra-rare event like xenon-124 decay, it has a good chance of also spotting a dark matter particle colliding with one of the xenon atoms inside that tank.

Now all the XENON team needs to do is wait for it.

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