Exosia is a subspace layer, the home of the Caretaker's mate, Suspiria. It was described as a realm of pure thought and energy. Suspiria invited the most mentally developed of her Ocampa followers to join her there. (VOY: "Cold Fire")
Background
In the non-canon String Theory trilogy, it was revealed that, while Exosia is the natural realm of the Nacene, Suspiria and the Caretaker, like numerous other Nacene, are actually exiles from Exosia, who sought to explore the universe rather than care for the quantum strings that exist in Exosia. After a battle between Voyager and the exiled Nacene, Kes, Q and the Doctor are able to oversee the birth of a Nacene-Ocampa hybrid, who stabilizes reality while Kes helps the Nacene evolve to the next stage of their development by absorbing the photons into themselves.