The galactic rim refers to the region of space at the edge of the Milky Way galaxy. With its sparse stellar population, the rim is largely uninhabited compared to the rest of the galaxy. The Delta Vega sector is one of the regions along the galactic rim. Beyond that, at the very edge of the rim, lies the galactic barrier, which (presumably) encompasses the entire circumference of the Milky Way.
Thought of by some cultures as "the end of everything familiar," an air of mystery surrounded the galactic rim. Most of the Class M planets discovered in the region were long-dead by the modern era, and some contained the ruins of ancient civilizations. These long-vanished races were thought to have gone "beyond the rim."
During the 23rd and 24th centuries, the Federation Starfleet launched several missions to explore along the galactic rim. The USS Pendragon and USS Sojourner were two starships given such a task in the 2380s. Starbase 237 was a Starfleet facility near the galactic rim. (Star Trek: Pendragon: "Whispers As Loud As Thunder", "Land of My Sojourn")
In the 2350s, young Timothy Sinclair wandered the galactic rim. During that time, he encountered "mysteries" that he could still not comprehend thirty years later. The Lalairu species group also made pilgrimages to the rim. (Star Trek: Pendragon: "Past Watchful Dragons", "Of Shadows and Starlight")
During a survey of the galactic rim in 2364, the USS Cerberus was destroyed by an unknown force. There were only nine survivors, including Timothy Sinclair and Jack Hayes. (Star Trek: Pendragon: "Wings As Eagles", et al)