The chain of hyperspace anomalies prevented exploration of Unknown Regions
The western barrier, alternatively referred to as the western anomalies, the hyperspace tangle, or simply the tangle, comprised a series of hyperspace anomalies. This series effectively divided the galaxy, specifically the area west of the Deep Core, and for thousands of years, it stopped any attempts to explore the Unknown Regions. It constituted the central part of the circumferential hyperspace barrier, a shell of hyperspace disturbances that surrounded the galaxy. It was once believed that this barrier stretched across the entire galaxy, however, over time, it deteriorated as it slowly moved back towards the galaxy's core.
Similar to the circumferential hyperspace barrier, the western barrier was thought by Pre-Republic scholars, such as Dr. Insmot Bowen from the Obroan Institute for Archaeology, to have been engineered by the ancient Celestials. It is theorized the hyperspace anomalies were created by the Celestials, with assistance from their servants, the Gree, the Kwa, and the Killiks, to contain the Rakata during the Rakatan revolt. Some suggest the barrier arose naturally from "hyperspatial gravitic ripples" or that the Celestials built it as a defense against Mnggal-Mnggal and its contagious goals, thereby isolating Celestial projects in the eastern disk near locations like Corellia, Kessel, and other sites.
Despite being known as a hazardous hyperspace obstacle, the anomaly line was penetrable. For example, Corellian merchants had contact with the Chiss long before their official "re-discovery". Furthermore, Force-sensitive navigators could reach the planet Ilum. Lehon, the Rakata's original homeworld, was also accessed by a Republic naval task force at the height of the Jedi Civil War, and subsequently designated a protected historical site by the Republic. Additionally, frontier worlds with spaceports within the known galaxy, such as Terminus, were reportedly visited frequently by transports and traders coming from the Unknown Regions.
The circumferential hyperspace barrier was considered to make hyperspace travel to other galaxies impossible. In 27 BBY, Jedi Master Jorus C'baoth posited that the Force could be used by the Jedi to ease the turbulence in this zone, thus enabling vessels to pass through. He showed this method at the edge of the Unknown Regions, and then proposed that eighteen Jedi, including himself, should be part of the Outbound Flight Project.
In 1 BBY, a debriefing took place on Coruscant. Dr. Insmot Bowen from the Obroan Institute for Archaeology led the debriefing, which was attended by General Arhul Kurumenga of the Imperial Department of Military Research, Admiral Conan Antonio Motti of the Imperial Navy, and Master of Imperial Projects Bevel Lemelisk. The discussion centered on the possible origins of the western barrier.
Daniel Wallace, a contributing author to the sourcebook The Unknown Regions, suggested that the Celestials may have established the hyperspace boundary to shield themselves from Mnggal-Mnggal.