On the night of Oct. 15, 1991, a cosmic ray streaked across the sky over Utah carrying more energy than any particle seen before or since, Quanta Magazine reported.

It became known as the "Oh-My-God" particle. It carried about 320 exa-electron volts of energy, millions of times more than particles reach in the Large Hadron Collider, Quanta reported. Astronomer Phil Plait, writing in Scientific American, put it at 51 joules, which he wrote is about the same as a slow curveball, all packed into one subatomic particle.

The particle was caught by the University of Utah's Fly's Eye detector, an array of dozens of round mirrors set on a desert mountain, Quanta reported. Researcher Mengzhi "Steven" Luo had switched on its computers at dusk from an old trailer nearby. The event was spotted in the data later, Quanta reported.

"Nobody ever thought you could concentrate so much energy into a single particle before," University of Utah astrophysicist David Kieda told Quanta.

The particle posed a puzzle. Theory said particles above about 60 exa-electron volts should lose energy to leftover radiation from the big bang, so it must have come from somewhere fairly close, Quanta reported. But scientists saw nothing in that direction that could have produced it. "It's like you've got a gorilla in your backyard throwing bowling balls at you, but he's invisible," Kieda said.

Plait wrote that no known source in the Milky Way could do this, so it very likely came from another galaxy. He wrote that it may have been a heavier nucleus, such as iron, rather than a proton, which could help it make the long trip.

The mystery pushed scientists to build bigger detectors, which by 2015 had recorded a few hundred cosmic rays above that 60 exa-electron volt limit, though none as strong, Quanta reported.

What we know: A Utah detector recorded the most energetic cosmic ray ever seen on Oct. 15, 1991.
What we don't: What kind of particle it was and what object in space sent it.