If you weren't looking at the constellation Leo very early on Saturday morning, you probably missed the brightest explosion NASA scientists have ever observed. It was three times as bright as the next-brightest explosion, and a ridiculous, basically unimaginable 35 billion times brighter than visible light.
The explosion was a gamma-ray burst, or GRB, a type of event that's the brightest we know about in the universe. During a supernova, in which a massive star collapses into a black hole, neutron star, or quark star, sometimes a GRB is emitted. Nobody's exactly sure how GRBs happen, but they're observed during a supernova and consist of a tightly focused, narrow beam of radiation, moving at speeds about as close as you can get to the speed of light without exceeding it. Imagine compressing a weird space-apple from all sides until a jet of space-apple juice explodes out of a tiny point before the apple turns into a black apple hole. (This is not a perfect analogy.)
They're also often the source of the perennial superlative of "farthest thing we've ever seen," since they're so bright and move so fast.
This one, about 3.6 billion light-years away, was observed by Fermi's Gamma-ray Burst Monitor, and was subsequently seen by just about any ground-based observation unit that was pointed anywhere near the constellation Leo. Astronomers often use GRBs to find the supernova from which they emitted; the GRB is so bright that it's a useful way to pinpoint where a supernova may have happened. NASA says they expect to find that supernova within a couple of weeks.