When the first interstellar explorers reached Gliese 667 Cc and found the elaborate underground cities beneath its surface, humanity rejoiced at discovering proof that we were not alone in the universe. Although the inhabitants of the planet had gone extinct long ago, the fact that they had once lived seemed to be a clear sign that intelligent life must still thrive elsewhere in our galaxy.
As we searched world after world, we found other empty cities, other abandoned relics, other memorials to forgotten civilizations. At first, we dismissed this as pure chance, a quirk of cosmic scheduling that brought humanity to the stars during a brief period of galactic silence. This simple explanation failed when we found that many of these dead worlds did not bear the scars of global war, pandemic, or famine. The populations of these planets had vanished, leaving behind no corpses other than any ancestral remains they may have preserved due to their own customs.
The Sigma Event struck different worlds at different times. The most recent suspected occurrence was just over a century ago, and the oldest we know of happened fifty thousand years ago. On any given planet, evidence suggests the inhabitants had been going about their usual business and then, all at once, just disappeared. Explorers found vehicles abandoned in transit and tools dropped in the middle of use. Ancient recording devices that survived the passage of time and that might have observed this event have been of no help, as all such recordings show nothing but inexplicable random data for a period of fifteen minutes to a half-hour surrounding the event.
Hundreds of theories have arisen to explain these disappearances, from a galaxy-wide nanotech infestation to some sort of spontaneous spiritual transcendence. Though the Xenohistory Corps is still unable to provide an answer to this mystery, they have uncovered evidence that humaans were not the first people in the galaxy to try to understand the Sigma Event.