A highly advanced species near the centre of the galaxy who explore by luring other travellers to them rather than journeying out themselves.
The Cytherians are an advanced species living near the centre of the Milky Way. Rather than travel the galaxy themselves, they send out probes that reprogram the ships or minds of other species, giving them the means to make the journey to the Cytherians, who then exchange knowledge with their guests.
In 2367, in The Nth Degree, a Cytherian probe encounters the USS Enterprise-D near the Argus Array and transforms Reginald Barclay, raising his intelligence to an extraordinary level. Barclay takes control of the ship's computer and moves the Enterprise across tens of thousands of light years in moments, alarming the crew, who fear he has been taken over by a hostile force. When they arrive, a Cytherian appears as a giant head on the viewscreen and explains the peaceful purpose of the encounter.
The Enterprise spends ten days with the Cytherians, gathering knowledge that will take decades to study, before being returned. Barclay reverts to normal but keeps his memories, and the episode offers a gentler take on the idea of a being with vast power.