Astronomers have identified what they think are the 514 most energetic objects in the observable universe.
The unnerving problem is they don't know what 65 of them are.
Found using the Fermi Space Telescope, which scans the sky for gamma rays every three hours, the objects are all throwing off more than 10 gigaelectronvolts of gamma ray energy.
"What that means is that we know it's a gamma-ray source, but we don't know what kind of source," David Paneque of the Max Planck Institute for Physics in Munich, Germany, told BBC News.
"We can't associate it with a radio object, with an optical object. It might be actually a new class of object - something that only emits in gamma rays."
Among the ideas currently proposed are that the mysterious sources of energy could be 'dark galaxies' made of dark matter - which is widely suspected to make up the majority of the universe's mass, but which has never been observed.
But it is also possible the objects could be something totally new - and the team behind it says they will keep watching the skies until they can figure out what they are.