Though prior to the Judgment War, there were no confirmed encounters with sentient alien life, possible indicators of previously-existing sentient races have been found across known space. Numerous indigenous plant and animal races have been found on countless worlds, but none seemed to possess any truly sentient life.
Below are listed some of the more well-known discoveries made over the history of the Solar Alliance that have been popularized as “proof” of previously-existing alien races.
The Oblivion Gulf
This patch of space in the Perseus Arm was first discovered by remote observatories in 2816, and later formally explored in 2967. It took nearly a full century to map its extent. The Gulf is unique because it is empty. It is utterly devoid of anything larger than cosmic dust, and even this is in far lower proportions than elsewhere in the galaxy.
The Oblivion Gulf is nearly ten light-years in diameter, and a complete mapping of its shape has revealed it to be similarly shaped to a powerful explosion, albeit on a scale far greater than anything conceivable by man. Whatever happened in the past, it seems to have annihilated all matter within its reach, leaving a true vacuum for dust and matter to slowly fill back in.
Perhaps the Gulf was the result of a terrible weapon that annihilated all trace of its creator. Perhaps it was an exceptionally powerful hypernova that exploded aeons ago. There is, unfortunately, not even a scrap of matter, not even a trace of residual energy, to analyze and try to discern the truth.
The Ghosts of Iolchis
"They say that on Iolchis, that barren world on the edge of the Devros sector, if you travel far from the cities, into the wastelands that cover much of the planet, and look out during an ion storm, you can see ghosts. But only if you have green eyes. Some quirk of genetics or some other factor means only green eyes are sensitive to the energies that appear as dancing after-images and shadows of buildings long gone. The red sands disappear beneath ghostly echoes of greens, blues, and whites. You can walk among the ghosts, even walk through them. They don't seem to ever notice visitors.
No one's really sure what they are. Spirits of the dead? Spirits of the living temporally displaced ever so slightly? Are they future inhabitants of Iolchis, or all that's left of a long-dead race? As long as people have lived on Iolchis, the ghosts have appeared. How long have they been going about their shadowed lives before we came here? All attempts to communicate have failed; likewise with all attempts to figure out what they are or how they appear, beyond theories of the ion storms triggering a weakening in reality, pushing two points in time closer together. But other planets have ion storms, and no ghosts to go with them. Who can say what is special about this world. We can but go on our lives, and dream of ghosts."
-Data entry, Codex Solaria, author unknown