In Star Trek, the galactic barrier is an energy field composed of negative energy surrounding the rim of the Milky Way Galaxy. It appears as a purple/pink band of energy. No form of transmission can penetrate the barrier. (TOS: "By Any Other Name") Warp travel through the barrier causes extreme sensory distortions. (TOS: "Is There in Truth No Beauty?") The barrier has been encountered on several occasions by Earth and Starfleet vessels. The composition of the barrier was initially totally unknown but in TOS: "By Any Other Name" it is directly and clearly stated to be in fact negative energy.

The first encounter involving an Earth vessel occurred in 2065 with the SS Valiant. The ship was swept into the barrier by a magnetic storm. Subsequent events resulted in the destruction of the Valiant and the loss of all hands, leaving the barrier's formal discovery to wait two hundred more years. In 2265, the USS Enterprise under the command of Captain Kirk discovered the recorder-marker ejected from the Valiant. Decoding its message, they retraced the Valiant's path, being almost destroyed in the process. At that time the barrier did not register at all on the Enterprise's sensors, although the ship's deflectors did react to it.

In 2268, Kelvans from the Andromeda Galaxy entered the Milky Way Galaxy through the barrier. Their advanced multigenerational ships were damaged by the barrier and subsequently destroyed. The Kelvan survivors of one ship required a new vessel for the journey home. They hijacked the USS Enterprise and refitted it to survive the barrier and the intergalactic journey.(TOS: "By Any Other Name")

Later that year, the Enterprise left the Milky Way Galaxy and was stranded inside a space-time void deep within the barrier. No sensor data or reference points were available to exit the void in the right direction. Fortunately Kollos was able to help pilot the ship out. (TOS: "Is There in Truth No Beauty?")

In actual fact, there is no purple band of 'negative energy' surrounding the rim of the Milky Way called the Galactic Barrier. However, the Milky Way is surrounded by the Galactic Halo, comprising of dark matter, star clusters, clouds of gas and a few lone old stars. While the bulk of our galaxy's stars are concentrated in a fairly flat disk and a bulbous central region, the halo is the first thing an intergalactic traveler would encounter upon approaching our home galaxy. The halo begins at the edge of the disk around 65,000 light years from the galactic center and may extend out as far as 300,000 light years from the center of the galaxy.