The universe is more than 13 billion years old. Multiply this number by a trillion and that is the half-life of a xenon-124 atom, which is the time it takes for a group of xenon-124 to diminish by half.

For the first time ever, scientists were able to observe this decaying process in a near-impossible feat.

"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, coauthor and an assistant professor of physics at Rensselaer Polytechnic Institute, in a report from the university. "It's an amazing to have witnessed this process, and it says that our detector can measure the rarest thing ever recorded."

Amazingly, the scientists were able to pull it off using an instrument that's designed to search the universe for another elusive sight: dark matter.

Dark Matter Hunter Captures The Decay Of Xenon-124

Buried deep in the mountains of Italy is XENON1T, a massive tank of pure liquid xenon shielded from radiation. It's a machine meant to search for dark matter through the particles' interactions with the xenon, but it hasn't been successful with this goal just yet.

Instead, the XENON Collaboration has captured another ultra-rare particle interaction: the radioactive decay of the xenon-124 atom into a tellurium 124 atom, which the team shared in a paper published in the journal Nature.

For most elements, radioactive decay occurs when one electron is pulled into the nucleus to convert a proton into a neutron. However, in a xenon atom, a proton must absorb two electrons to convert into a neutron. It's a very rare event known as a "double-electron capture."

The Rarest Event Recorded

Brown explained that a double-electron capture only occurs when two of the electrons are right beside the nucleus at the exact right time.

"[It's] a rare thing multiplied by another rare thing, making it ultra-rare," he added.

As soon as this double-electron capture occurred in the machine, instruments detected the signal of electrons re-arranging to fill in for the electrons that have been absorbed.

It's the first time scientists have been able to directly observe a radioactive decay of a xenon-124 atom. By doing so, the team were also able to precisely measure its half-life as about 18 sextillion years, a trillion times the age of the universe.

"If you had 100 xenon-124 atoms when the dinosaurs went extinct 65 million years ago, statistically speaking, all 100 of them would still be there today," Christian Wittweg, a doctoral candidate from the University of Münster who is one of the 106 scientists part of the XENON Collaboration, described to Live Science.