THE BARRIERS
The Milky Way Galaxy is delineated by a pair of energy barriers. The Galactic Barrier is located at the rim of the galaxy. It is an energy field of great magnitude which renders passage by ship extraordinarily difficult. Furthermore, in 2266, it was shown to boost the psionic abilities of certain humanoids to almost godlike levels. Because of this, travel to the Galactic Barrier is interdicted by Starfleet. The Galactic Barrier fortunately serves to keep aliens from other galaxies _out_ as well as us _in_, although not always (see below). The Galactic Barrier at the rim of the galaxy is matched by a Great Barrier at the center of the galaxy, which was thought to be impenetrable, at least until the USS Enterprise crossed it in 2287. It did not seem to boost psionic abilities.

THE ANDROMEDA GALAXY
Very few races have made it across the Galactic Barrier from outside the Milky Way. Even with warp drives, the massive distances discourage intergalactic travel. One such race who made it were the Kelvans, a highly advanced race from the Andromeda Galaxy. In form they were non-humanoids with a hundred tentacles and no tactile perceptions or emotions, although this was not initially discovered since they were capable of taking human shape. They revealed that their scientists had discovered that increasing levels of radiation from Andromeda's core would render their entire galaxy uninhabitable within ten millennia. The first group of Kelvans were pacified and settled within Federation space, but the next group who arrive might not be as easily dealt with....

Another species from the Andromeda Galaxy were a race of humanoid androids who, because they were immoral, presumably were able to survive such a long trip. They were created by a race of beings known as the Makers, and when the star of the Makers' homeworld went nova some one million years ago, the androids were stranded in their base in the Milky Way. When their Makers never returned, the androids were keen to serve the first intelligent life-form they encountered, which sadly turned out to be the Federation rogue, scoundrel, and criminal Harcourt Fenton Mudd. The androids intended to expand and smother the Federation beneath the iron hand of their "benevolent" service, but were quashed when the illogicality of humans defeated them. Nevertheless, they still exist, and they do possess star travel. And more might arrive yet.