The Cytherians were an advanced species who lived near the center of the Milky Way Galaxy. They were explorers and made contact with various other races. The manner in which they initiated first contact was atypical; they brought other races to them rather than traveling the galaxy in search of intelligent life.
In 2367, the USS Enterprise-D encountered an alien probe while attempting to repair the Argus Array. The probe interfaced with Lieutenant Barclay, and gave him the advanced knowledge needed to bring the Enterprise-D through a subspace distortion to the Cytherians, the presumed makers of the device. After the Enterprise-D had arrived at the Cytherians' homeworld, Barclay was returned to normal. (TNG: "The Nth Degree")