Hellsmouth Nebula

The Hellsmouth Nebula is a vast and hazardous region of space located between the Federation and Romulan borders. This region has long been recognized as a significant navigational hazard due to its unstable pockets of hydrogen and plasma, causing frequent explosions in rapid succession. Starfleet has classified it as a dangerous celestial formation, impeding the safe passage of starships.
Research outpost Chiron Station was established in 2346 to study the nebula and attempt to identify both an origin for the phenomenon as well as scan for safe passage to the other side.
In 2376 an Unknown Alien Dreadnaught created a hole in the nebula using an crude subspace engine that tears the fabric of space-time allowing navigation through the Hellsmouth and access to the Typhon Expanse on the other side.

2378 Update

The USS Nightingale was assigned with exploring and surveying the Typhon Expanse and in the process discovered numerous examples of an ancient and technologically advanced civilization called the Morghi. Studying the energy systems used reveals trace similarities between the Hellsmouth and this ancient tech leading to hypothesis that the Hellmouth was artificially created in order to isolate the Typhon Expanse from the rest of the galaxy. The motives behind such an act remain unclear.
Starfleet Command recommends further investigation and increased reconnaissance missions in this area to unravel the mysteries surrounding these phenomena.