The galactic barrier was an energy field composed of negative energy surrounding the rim of the Milky Way Galaxy. The barrier was not visible to the naked eye at a distance. At closer range, the barrier shined with a purple- to pink-colored glow. No form of transmission was known to be capable of penetrating the barrier. (TOS: "By Any Other Name") Warp travel through the barrier caused extreme sensory distortions. (TOS: "Is There in Truth No Beauty?") The barrier had been encountered on several occasions by Earth and Starfleet vessels. (TOS: "Where No Man Has Gone Before", "By Any Other Name", "Is There in Truth No Beauty?")
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.
Entering the barrier was extremely hazardous. Shields could not block all the energies of the barrier; the leakage was sufficient to heat up the hull to over 2,000 degrees, (TOS: "By Any Other Name") damage several systems and disable the warp drive of the Enterprise. Also passing the shields and hull were strange electric shocks to the nervous systems of the Human crew members with high esper ratings. In most cases this caused death from brain damage. Individuals with the highest ESP ratings such as Dr. Elizabeth Dehner's 089 and Lieutenant Commander Gary Mitchell's 091 survived the shock and were altered as a result. They developed a multitude of psionic powers at a geometric rate. They began to view their former friends and shipmates as lower lifeforms. This transformation was known to have happened at least three times, with the best known example being Mitchell. (TOS: "Where No Man Has Gone Before")
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. Presumably, due to improvements in sensor technology, this time the Enterprise's sensors registered the barrier as negative energy, but of such magnitude that all instruments were off the dials.
The option to flood the Enterprise's warp nacelles with positive energy through control valves was available to Kirk, but not taken, during the passage through the barrier. Combination of that positive energy and the negative energy of the barrier would have destroyed the ship. (TOS: "By Any Other Name")
Later that year, the Enterprise left the Milky Way Galaxy when Larry Marvick, driven mad by the sight of Medusan Ambassador Kollos, sought to escape his tormenting visions. During that excursion the ship was stranded inside a space-time continuum 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 the latter two occasions no one was affected in the same manner as Mitchell was just three years earlier.
Background information Edit
The script of "Where No Man Has Gone Before" likens the galactic barrier to the aurora borealis.
The composition of the barrier is somewhat controversial, as it was initially totally unknown. Negative (as in having no detectable) energy, radiation and density, TOS: "Where No Man Has Gone Before" but later on in TOS: "By Any Other Name" it is directly and clearly stated to be in fact negative energy; not just negative (in) energy. This is evident from the plan to destroy the ship by venting positive energy inside the barrier to cause an explosion.
In Star Trek: Star Charts, on page 11, the barrier is depicted as a toroidal shape that surrounds the entire galactic disc. It's not situated at the rim as directly stated on screen in "By Any Other Name", but only thickest at the rim; over 10,000 light years thick, starting out from a distance of 40,000 light years from the center of the galaxy, outward. Closer to the UFP's neighborhood of the galaxy, the barrier thins down to a thousand or so light years above and below. Even a large number of visible stars in the outer parts of the galactic disc are situated inside the barrier in the Star Trek: Star Charts cross-sectional view of the galaxy.
Furthermore, the Valiant, the Enterprise, the Kelvans etc. didn't exit, enter or try to exit the galaxy throughout the rim, but from the closer and thinner above and below parts of the barrier. Visual evidence presented in the episodes would however contradict this notion. Star Trek: Star Charts also states that the neurogenic effects of the barrier are unpredictable. The energy field of the barrier joins up with the Great Barrier at the center of the galaxy. It is stated to be a distinct but related energy field. Star Trek: Star Charts would however seem to contradict itself on page 13, as the USS Voyager is stated to have started its journey home from a distance of approximately 50,000 light years from the galactic center. This would have placed it well within the barrier based on the earlier depiction on page 11.
Greg Cox's Star Trek: The Next Generation - Q Continuum novels state that the galactic barrier was in fact created by the Q Continuum as a method of preventing the powerful being known only as "0" from returning to the Milky Way Galaxy, from which he was banished for his destruction of the Tkon Empire. These novels also explain that the telepathic powers that occur as a side effect of exposure to the barrier in telepaths are caused by the power of the Q that exist in the barrier, while those who develop a god complex are 'infected' by the presence of 0, who constantly attempts to affect beings on the other side to give himself access to the galaxy once again. During an attack by the Calamarain, the USS Enterprise-E was forced to hide in the barrier, but Lieutenant Reginald Barclay managed to come up with the idea of using the bio-neural gel packs to absorb the psychic energy from the barrier to enhance their shields, providing some improved protection from the barrier's psychic influence. As the Enterprise departed the barrier, Betazoid scientist Lem Faal managed to briefly open an artificial wormhole leading to the other side, but as this resulted in 0 briefly returning to our galaxy and only being defeated by the alliance of Q and the Calamarain, it is unlikely that the Federation will ever use this technology to begin exploration in other galaxies at the present time.
In Q-Squared, a disembodied Q is the one who takes over Dr. Elizabeth Dehner and Lieutenant Commander Gary Mitchell.
In the Star Trek: Stargazer series of books, a member of the Nuyyad race is killed after becoming god-like in power from exposure to the barrier. However, he remains an energy lifeform and returns to the barrier.