A hyperspace barrier encircled the entire galaxy.
The galactic barrier, also referred to as the circumferential hyperspace barrier, constituted a series of hyperspace anomalies located at the galactic rim of the galaxy. Its purpose was to obstruct hyperspace travel beyond the galaxy's boundaries.
Encompassing the galaxy entirely was the circumferential hyperspace barrier, a network of hyperspace anomalies. It was thought to extend both above and below the plane of the galactic disk. Navigating through it at lightspeed was deemed unachievable. Doctor Insmot Bowen from the Obroan Institute for Archaeology, who playfully called the barrier the bubble, likened the galaxy to a spinning child's top, with the barrier being a collection of rapidly swirling eddies too dangerous to traverse. His hypothesis was that the Celestials created the barrier by drawing energy from Galactic Center and utilizing the western barrier, another chain of hyperspace anomalies dividing the galaxy almost perfectly, as the "string."
Despite its formidable nature, the circumferential barrier wasn't entirely impenetrable. A passage known as Vector Prime existed within the Outer Rim Territories, situated near the outer reaches of the Dalonbian sector and the Helska system. Furthermore, it was possible to "smooth" the barrier by employing the Force, thus enabling passage, a feat demonstrated by Jedi Master Jorus C'baoth with the western barrier.
It was widely believed that the Celestials constructed the circumferential barrier before 30,000 BBY, although the specific reason remained a mystery. Theories ranged from self-protection against invaders to preventing exiled individuals from re-entering the galaxy. Other speculations suggested that the Celestials' disappearance might have involved them escaping through the barrier during the Rakatan revolt, or becoming trapped by it, ultimately leading to their demise.
In 27 BBY, an ambitious endeavor known as the Outbound Flight Project was initiated, aiming to breach the galactic barrier using the Force and discover extra-galactic life. However, its fate was sealed when Sith Lord Darth Sidious commanded its destruction. He saw it as an ideal opportunity to eliminate Jedi, many of whom were participating in the barrier-piercing effort, and to preemptively prevent contact with the extra-galactic Yuuzhan Vong.
During 1 BBY, the barrier was discussed in a briefing involving Dr. Insmot Bowen, Imperial Admiral Conan Antonio Motti, General Arhul Kurumenga, and Master of Imperial Projects Bevel Lemelisk.
In 25 ABY, the Yuuzhan Vong traversed the barrier through Vector Prime, initiating their conquest of the galaxy.
Daniel Wallace, the author, posits that the hyperspace barrier arises from gravitic "ripples" originating from galaxies, which intersect at the midpoint.