The Fermi Paradox
"Why haven't we found Aliens yet?" or "The Fermi Paradox" changed in 2117 when living Alien life was first discovered in the Aoi Si. This indicated that life should be common in the universe, after all if life was extremely rare what were the odds that we'd find some so close to earth?
Then more Aoi Si were found by the PRC a few years later in the opposite direction of those located in Kitzeh. Aoi Si appeared to be common. One could reasonably expect to find them on habitable planets every hundred lightyears or so.
The Aoi Si were an ancient race, having left their homeworld for the stars long before humanity. They were smart like humans, capable like humans... So why was their civilization crumbled into dust? With life so common in the universe, the Fermi Paradox suggests that some species should have colonized the entire galaxy by now. So where are they?
The question only grew further in 2181 with the discovery of the Grawla. The Aoi Si weren't some outliers, intelligent life was common. Further questions were raised in 2320 with the discovery of Azathoth showing that more technologically advanced life existed in the galaxy at some point.
So Humanity was left to wonder, how come no other alien life has colonized the galaxy with their massive headstart in time and technology?
Something out there must be stopping technologically advanced races from spreading throughout the Galaxy. Other races must have had millions of years to try and reach for the stars, and this Late Filter must have stopped all of them.
Something is out there.
And it doesn't like Civilizations that stray into space.