Exosia is the subspace domain of the Nacene, described in Cold Fire as a layer of pure thought and energy to which Suspiria promises to take Kes and her Ocampa followers.
Exosia is spoken of in the Voyager episode Cold Fire. Tanis, leader of the Ocampa living on Suspiria's array, tells Kes that it is a realm beyond ordinary matter where the Nacene exist as pure energy and where thought alone shapes reality.
Suspiria holds out the prospect of Exosia as the reward for Ocampa who master their psychokinetic gifts under her direction. Kes is tempted but repelled by the destructive side of the powers Tanis teaches her.
After her failed attack on Voyager, Suspiria vanishes back into Exosia, taking Tanis with her. The realm itself is never shown on screen, and Voyager does not encounter the Nacene again.