The Cytherians are the other species at the level 100 level in the Weibrand civilization scale.
They live near the center of the Milky Way. As explorers they make contact with many species by bringing them to their own location. Indeed, they do not travel. They are very interested in intelligent beings.
It is in 2367 that the USS Entrepise-D of Jean-Luc Picard crosses one of their individuals. The Cytherians had then given to Lieutenant Barclay an incommensurate mathematician’s mind that allowed him to create a subspacial distorsion in order to join one Cytherian. This one appeared via an interactive hologram on the bridge and reset Barclay’s brain. The information transferred to the team will take decades to be analyzed.
There is no more information. We don’t know how old is their civilization, what is their true form. Given their level of intelligence, it is possible that the Cytherians are as old as the Tkons, i.e. several hundred thousand of years or even millions of years. Riker in The Next Generation: Takedown indeed refers to them as the ancients.