The Unnoticed were a species created by a temporal paradox.
The Unnoticed were black and slimy, smelling of decay. Their thorax was suspended from a gas-filled sack by strips of flesh. They had spindly legs hanging from the trunk, between them suspended an open stomach sac, filled with digestive juices. Their slime-slicked, fur-covered bodies were infested with needle-jawed worms. Their gas-sacs flashed blue with energy, which was a means of communication. (PROSE: The Book of the Still)
They lived in Tent City, a city of tents made from superstring taffeta, which could withstand the conditions on the surface of a star. They kept a breeding colony of humanoid time-sensitives, which they used as canaries to warn them of any nearby planets with time travel capability or passing time travellers who could threaten their existence.
They used a wave interrupter to disable entire solar systems, which stopped any and all waves right down to the flow of electrons through a wire. They also had bombs capable of destroying an entire planet. (PROSE: The Book of the Still)
The Unnoticed were created by a temporal paradox. They evolved from the fused-together Lebensweltian criminals Darlow, Gimcrack and Svadhisthana, who were thrown millennia back in time by the energy released by the time differential when they touched one of their descendents, the Unnoticed.
Because their existence was a temporal anomaly, they knew that they should not exist and therefore did not have contact with any other race. They had no written or oral history because their existence was so precarious. For the same reason, they did not study themselves. They excised any evidence of their existence completely, destroying entire planets if necessary. They destroyed entire races they suspected of developing a legend of their existence.