In other words, things like meteorite impacts and nuclear detonations.
These are intense explosions happening in distant galaxies, which are very rare and energetic. The theory is that they happen when big stars collapse into neutron stars or black holes, in a supernova.
Think about that: It's an explosion that could happen on the OTHER SIDE OF THE GALAXY and still kill us all.
Because we couldn't see them until we had gamma-ray detectors in orbit. The atmosphere absorbs most of the gamma rays.
Nuclear explosions produce a bunch of gamma rays, so we could use gamma-ray detecting satellites to find out if the soviets were doing secret tests.
They just merged this functionality into some satellites we were already launching:
The Global Positioning System satellites!
The first gamma-ray bursts were written off as malfunctions or spurious cosmic events, but there's at least one time the detectors went off that hasn't been tracked back to a known above-ground nuclear test: The Vela Hotel Incident.
It has to do with how the explosion's shockwave travels through the atmosphere. There's detectors for this on most tanks and bunkers, and on satellites.
All the others were blocked by weather or aiming a different way, almost like someone timed the test to when it couldn't be seen...