- You may also be looking for Great Rift.
The USS Pioneer attempting to enter the Great Barrier Rift from Star Trek: Pioneer
The Great Barrier Rift is a greenish hued gaseous cloud which separates the Oralian sector from the space controlled by the warlike Kai Imperium. No ship, that has not had its shields specially modified, can pass through it unless it is of Kai origin. No Starfleet vessel has ever been able to cross it. The only ship to enter the rift was the USS Pioneer under the command of Captain Benjamin Kelsoe. The only known inhabitants of the rift are a species known as the Camelyons, who the Kai call Rift Ghosts. (Star Trek: Pioneer)
According to the Kobalians, the Great Barrier Rift is made up of the remnants of the planet of Oral, which according to legend was the homeworld of the Oralians. ("The Bonding")