The Ancients were a very old race. That much was known
They were also a very mature and peaceful people.
They had got this way by being very successful at avoiding contact with other space faring, but more aggressive, races.

They had been invaded once, a very long time ago.
The invaders had eventually left as they couldn't stand the horrible blue sky.
Or was it because they had news of a new planet, with an abundance of sexually demanding females?

Anyway, from some of the fragmented files, it appeared that the Ancients had thereafter developed the ability to mask their life signs from passing space ships.
And an ideal thing for survivors trying to hide from really bad asssed aliens.