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.