While en route, the Enterprise had stopped to view the Beta Niobe supernova where it encountered an alien ship heading directly into the nova at a speed of Warp 36. Fearing for the life of the sole occupant of the alien ship, Kirk ordered his crew to engage the tractor beam so as to slow or even stop the vessel's course. The tactic failed and the alien ship ended up dragging the Enterprise into the nova and then into the antimatter universe where black stars shone in a white void.
In the antimatter universe, Kirk's crew quickly came to realize that the normal processes of their ship, as well as their bodies, worked in reverse. For example, the Enterprise's controls all worked in reverse, the ship traveled backwards through space, and the ship's chronometers all ran in reverse. Spock also commented that the crew themselves were growing younger in age. He would later theorize that the inhabitants of this antimatter universe were born in old age and would die in their infancy; essentially their descendants were born before them, and their ancestors were born after them. The brains of the crew also worked in reverse allowing them to understand language in the antimatter universe without the use of the universal translator.
Upon arriving in the antimatter universe, the pilot of the alien vessel, Karla Five, was contacted by the Enterprise and she offered the assistance of her world's scientists in getting Kirk and his crew back to their own universe. Karla Five escorted the Enterprise to Arret, her homeworld, and whose location in the antimatter universe corresponded with Earth in the normal universe.
Earlier, Karla Five had revealed her theory that stars dying in the normal universe, and stars being born in the antimatter universe acted as gateways between the two so long as they occupied the same space in both universes. It was the dying Beta Niobe and the antimatter universe's newly born star, Amphion, which had created the initial gateway for the Enterprise to enter the antimatter universe. With this information, the crew of the Enterprise then sought out two stars occupying the same space in both universes as a means to get home. (TAS episode: "The Counter-Clock Incident")
Billions of years ago, a Maltusian scientist named Krona, performed a forbidden experiment in an effort to see the origin of the universe. His irresponsible actions disrupted the process of cosmic creation, resulting in an additional, opposite universe.
The Antimatter Universe is also the location of the planet known as Qward. The Weaponers of Qward - devoted adversaries of the Guardians of the Universe have frequently crossed swords with their dimensional counterparts, the Green Lantern Corps. To aid them in their efforts to destroy the Guardians, they have frequently relied upon the aid of a former Corps member, Sinestro.
The cosmic being known as the Anti-Monitor is also indigenous to the Anti-Matter Universe. An analog to the universal Monitor, the Anti-Monitor attempted to destroy all reality with a wave of pure antimatter energy. This ambitious attack culminated in what has become known as the Crisis on Infinite Earths.
During the crisis, it was discovered that Alexander Luthor, the orphaned son of Earth-Three's Lex Luthor and Lois Lane was a being comprised of both Antimatter and normal matter. As such, he could function equally in both realities, without undue stress.
The Antimatter Universe consists of three million worlds, two million of which contain sentient life.
After the events of Infinite Crisis, which resulted in the recreation of a new separate Multiverse that contains 52 individual realities, there appear to be at least two distinct antimatter realities: