Determining the absolute rarest event in the universe is a challenging task, as it depends on the definition of "event" and the scale at which we're considering it. However, one contender for the rarest event ever observed is the radioactive decay of xenon-124, witnessed by a dark matter detector in 2019. This specific decay, known as "two-neutrino double electron capture", has a half-life of trillions of years, meaning it's incredibly unlikely to occur in any given atom.
While other events like supernovae or gamma-ray bursts are rarer in occurrence, they are still observable phenomena within the vastness of the cosmos. The xenon-124 decay, however, stands out due to its extremely long half-life, making it an incredibly rare event at the atomic level.
