The Typhon Expanse is a region of space in the Beta Quadrant prone to temporal distortions and other exotic phenomena. Parts of the Expanse lie in the Typhon, Pi Hydrae and Fries-Posnikoff sectors. Sector 221-G was nearby, as was the Benecia sector. (TNG: "Cause and Effect"; TNG novel: Ship of the Line; Star Trek: New Frontier novel: House of Cards)
In 2260s, the closest Starfleet outpost to the Typhon Expanse was Starbase 12 in the Fries-Posnikoff sector. By the late 24th century, the Federation facilities Deep Space 4, Deep Space 5 and Starbase 185 were located near the Expanse. (TNG novel: Ship of the Line; Typhon Station)
The USS Bozeman entered the Expanse in 2278 and did not return. Starfleet had presumed the Bozeman lost, but, in 2368, the vessel emerged from a temporal rift and collided with the USS Enterprise-D, destroying both ships and creating a temporal causality loop.
The loop repeated several times until Enterprise-D second officer Data sent a short message into the next loop, and, after several attempts, finally stopped the loop, saving both the Enterprise-D and Bozeman. (TNG: "Cause and Effect")
Later that same year, Helena Bourget undertook a medical mission for the Arkrand Society and did not contact any members of her family for over four months. As one of the Starfleet vessels assigned to explore the Expanse, the USS Huntly discovered her and her ship safe and sound, accompanying her to Starbase Expanse 3. (Star Trek: The Cantabrian Expeditions: "Breaking Benjamin")
By 2372, Starfleet devoted more resources to exploring and colonising the Typhon Expanse and the area surrounding it. The USS Cantabrian was one of the starships assigned to the area around the Typhon Expanse, and Starfleet took over and built several starbases in the area, including Starbase Expanse 3, Starbase Expanse 4 and Starbase Expanse 12. The Devil's Cradle was also on the edge of the Typhon Expanse. (Star Trek: The Cantabrian Expeditions: "Catalyst, Part One", "Catalyst, Part Two", "An Innocent Time")
In 2381, the Expanse was the location of clandestine meetings between the Breen Confederacy, Romulan Star Empire, Gorn Hegemony, Tzenkethi Coalition, Holy Order of the Kinshaya and the Tholian Assembly that led to the creation of the Typhon Pact alliance. The Expanse was then claimed for the Pact. (ST novel: A Singular Destiny, Star Trek: Typhon Pact novel: Plagues of Night)