An antimatter universe was a type of parallel universe on a different dimensional plane.
These universes were the antimatter opposites of the familiar positive matter universes. In general, matter from a positive matter universe coming into contact with antimatter from an antimatter universe would not result in an annihilation of both particles.
Minus universe
The minus universe was the home of anti-Lazarus, a man whose civilization created a negative magnetic corridor to safely connect with the positive matter universe.
The universes closely resembled each other, identical particles, individuals and planets existed in both universes, where one was composed of matter, the corresponding other was composed of antimatter. Only if identical particle counterparts from the different universes came into contact inside either universe they would annihilate each other and start a chain reaction that would annihilate the universe, where the event took place, as a whole. As long as this type of contact was averted, visitations could be made.
Within the negative magnetic corridors, an alternative warp allows safe interaction with counterpart elements of both universes. If a person with a counterpart were to step through to either side together, only one entity would exist, but the entity would periodically change places with its counterpart distorting the physical laws of that universe while the change occurs. (TOS: "The Alternative Factor")
Reverse universe
In 2270, while transporting Commodore Robert April to his retirement ceremony on Babel, USS Enterprise attempted to stop an alien starship heading for the Beta Niobe supernova at warp 36. While trying to rescue the ship, the Enterprise was dragged inside the nova and into the reverse universe.
In the reverse universe, the inhabitants of the planet Arret began their life at an old age and slowly grew younger. Brain functions, language and operational controls worked in reverse of what they did in the positive matter universe, as the flow of time was reversed. Beginning a process ended it and starting it stoped it. Space was white and full of black stars. All though the universes had totally different timelines, the planets and stars of the Milky Way Galaxy at least were located in exactly the same places.
During a short visit to the reverse universe lifeforms would grow younger at a normal but reverse rate. Spending more time in this dimensional realm, the rate of reverse growth increased to a rate where lifeforms would reverse grow to infants from adulthood within minutes and die in reverse birth. It is possible to use molecular structure records in the transporter system to restore the physical age. (TAS: "The Counter-Clock Incident")
- The novelization of "The Counter Clock Incident", written by Alan Dean Foster, showed that the reverse universe was in fact, an alien fabrication and that the Enterprise had never really left the positive matter universe.