The Typhon Expanse was a largely unexplored region of space discovered in 2277 by Starfleet, as of 2368, had yet to be explored by any Starfleet vessel.
Many vessels that entered the expanse were reported missing, and so, in 2287, after the loss of Starfleet's twelfth starship to enter the expanse, Starfleet abandoned exploration voyages that passed in or near the expanse. It was commonly accepted that the ships had been destroyed by Romulan warbirds, due to its proximity to the Romulan Neutral Zone.
However, in 2368, the USS Enterprise-D embarked on an exploratory mission in this sector, when it encountered a temporal distortion from which the USS Bozeman emerged, having been transported through time from the year 2278. Colliding with the Bozeman, the Enterprise was subsequently destroyed, and the resultant explosion, being in such close proximity to the distortion, ruptured the spacetime continuum. As a result, both ships became trapped in a temporal causality loop in which the Enterprise repeatedly encountered the Bozeman and was destroyed. Eventually, the Enterprise crew was able to determine the nature of the loop and avoid colliding with the Bozeman, thereby freeing both vessels from the loop.
In 2381, the Typhon Expanse was the location where the Breen Confederacy, Gorn Hegemony, Holy Order of the Kinshaya, Romulan Star Empire, Tholian Assembly, and Tzenkethi Coalition negotiated a political, military and economic alliance, known as the Typhon Pact.
Starbase 240 was located in the Typhon Expanse, the starbase was built there in 2383.