The Typhon Expanse was a region of space located in the Fries Posnikov and Typhon sectors of the Beta Quadrant and was well-known for its spectacular celestial phenomena, 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. (Star Trek: Intrepid)
In 2278, the Soyuz-class starship, USS Bozeman, was assigned to the Expanse on routine patrol. At the same time, several other starships were dispatched to the nearby Benecia sector to counter a fleet detected on the Klingon side of the border. That fleet proved to be a tactical feint, as the real attack force -- a single, over-built and over-armed cruiser, the Sosoy Tuj under Captain Kozara -- attempted to slip through the Expanse and attack nearby Starbase 12. The Bozeman managed to detect Kozara's ship and engaged it (even though overmatched). After dropping a warning buoy and leading the Sosoy Tuj out of jamming range, the Bozeman disappeared without a trace, entering a temporal causality loop. Its actions, however, allowed Starbase 12 to go on full alert and recall the fleet from Benecia Sector, effectively thwarting the Klingon attack plan. (TNG novel: Ship of the Line)
Ninety years later, the USS Enterprise-D entered the region and experienced the same causality loop, leading to a collision with the Bozeman and destroying the Enterprise repeated times as the loop repeated. Thankfully, Lieutenant Commander Data managed to solve the situation and both starships were able to exit the loop. As a result, the Bozeman was over 90 years out of time, and the Enterprise was 17 days out of time. (TNG episode: "Cause and Effect")
A supplier to Quark operated in the Typhon Expanse. (DS9 novel: Ascendance)
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. (ST novel: A Singular Destiny)