In the original Star Trek series, 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. In the episode "By Any Other Name", the USS Enterprise entered the Galactic Barrier and encountered a species from the Andromeda Galaxy, the Kelvans. They hijacked the Enterprise and refitted it to survive the barrier and the intergalactic journey back to their home galaxy. In another episode, "Is There in Truth No Beauty?", 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.

In astronomy, there is no purple band of 'negative energy' surrounding the rim of the Milky Way. 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.

The inner, visible part of the halo is occupied by objects such as globular clusters and old, individual stars. Beyond this is a much larger region, called the dark halo or extended halo, containing large amounts of dark matter, the presence of which is revealed by its gravitational effect on the galaxy's rotation. The nature of galactic dark matter is still undetermined but the most popular theory, supported by some recent observations, is that the dark halo is home to vast numbers of small, unseen bodies known as MACHOs (Massive Astronomical Compact Halo Objects).