The Unremembered Empire

I am not attempting to disparage the works of the other sapient races that humanity has encountered since the Paradise War when I say that the Unremembered Empire outclassed us all, and outclasses us still. A frustrating notion, to be sure, when we can't even determine when they were last walking around. Every new insight gleamed seems to suggest their empire was older and around longer than the last.

How does an empire rise to cover the whole of the galaxy, only to vanish without anyone around them noticing them in the first place?

That is the question that surrounds the Unremembered Empire. The ruins of what they built, greater than anything any sapient species has built now, dot the galaxy, not even hidden from view. Carbon dating and radiology indicate that this mysterious empire existed even as other sapients were building their own amongst the stars, yet no known species seemingly encountered this potent force.

What they left behind can seemingly be organized into two varieties, the biological and the technological. Some of their ruins seem built from plants, or bones, grown to shape and need, containing organisms more efficient and with better design than evolution could ever dream of. Others, found on other planets completely, made from black crystal and unidentifiable metal and housing ships and humanoid forms wrought from the same material.

Both biological and technological artifacts seem to defy logic and the laws of physics as casual as one might breath. These artifacts, of all shapes and sizes, are incredibly sought after by everyone; from pirate factions to research organizations to the most powerful governments of the known galaxy.