Galactic Barrier

The Galactic Barrier is just what it sounds like. In the 23rd Century, Captain James T. Kirk took the USS Enterprise through the barrier, which resulted in two of his crew gaining incredible psychic powers. Section 31 passed this information on to Dr. Janet Corbair, who was able to recreate the energies of the barrier and use them to bombard Ronnie Harper, resulting in Ronnie also gaining powers. ("The Khan Job" [TOS]).

By the 26th Century, Starfleet had developed a way to created a passable portal through the barrier at a station called the Barrier Observation Outpost. The USS Anomaly used this portal to leave for the Andromeda Galaxy and return to the Milky Way. ("Do You Have Anything To Declare?", "Disassociation" [BG])