“We actually saw this decay happen. It’s the longest, slowest process that has ever been directly observed, and our dark matter detector was sensitive enough to measure it,” said Ethan Brown, an assistant professor of physics at Rensselaer Polytechnic Institute about a process that takes more than one trillion times longer than the age of the universe. “It’s amazing to have witnessed this process, and it says that our detector can measure the rarest thing ever recorded.” Detected in the Search for Dark Matter The XENON Collaboration research team did it with an instrument built to find the most elusive particle in the universe—dark matter. In a paper published in the journal Nature, researchers announced that they have observed the radioactive decay of xenon-124, which has a half-life of 1.8 X 10^22 years.