The Galatic Barrier was first discovered by the S.S. Valiant circa 2065. At that time it was determined to be of natural causes and extending the perimeter of the galaxy. The Barrier was later found to embrace a section of the spiral arm, and not the whole of the galaxy. The USS Enterpise also visited the Galact Barrier in the 2260's and discovered high psionic reactions to individuals exposed to the Barrier's energy. It has been uncertain why the barrier only affected certain persons and not the whole crew.
With this reason in mind, the Galactic Barrier has been off-limits to all Starfleet ships except those delegated to exploring it from a safe distance. The Barrier had been speculated to be artificial in origin.
In 2384, the Klingon Empire sent out the Eigth Surveyor Mission to scout along the Barrier. Upon discovering a hole in the Barrier, they found an array transmitter of alien design. The Klingons then dropped a communications buoy and ventured beyond the Barrier. The 8th Surveyor Fleet has not been heard of since.
According to Starfleet Intelligence, in the past few years, the Tholians have alerted warships to the Barrier along their border. It has been undetermined the outcome of these responces and why they took place.
Starfleet Exploratory and the Military Reserves are developing a joint mission to trace the steps of the Klingon Surveyors in hopes of finding out what happened.
Author's Note: This information is strickly the invention of my own and does not reflect the ideas and opinions of the producers, writters and staff envolved with developing Star Trek.