"Exosia" was the name of a subspace layer in which the Nacene known as "Suspiria" resided. From there, she telepathically communicated with her Ocampa followers who lived on her array in the Delta Quadrant. The Ocampa Tanis described it as "a place of pure thought, pure energy... a place of the mind", and claimed that she might invite Ocampa to join her there.
In 2372, the USS Voyager encountered the array, and made contact with the Ocampa living there. Tanis, pretending to help the Voyager crew, sent a signal to Suspiria, who arrived in the normal spacetime continuum via subspace rupture. Suspiria subsequently attempted to destroy Voyager, but Captain Janeway was able to counter her attack with a sporocystian toxin, forcing Suspiria to retreat back to her realm. (VOY: "Cold Fire")