These universes were the negative antimatter opposites of the familiar positive matter universes. While in the prime universe things were composed of matter, the corresponding things in the negative universes were composed of antimatter. In general though, matter from a positive universe coming into contact with antimatter from a negative universe would not result in the annihilation of both particles. (TOS: "The Alternative Factor"; TAS: "The Counter-Clock Incident")
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 the exact identical particle counterparts from the different universes came into contact inside either universe they would annihilate each other. This would 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, safe visitations could be made.
Within the negative magnetic corridor, an alternative warp allowed 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. The entity would periodically change places with its counterpart distorting the physical laws of that universe while the change occurred. The effect included, for a split second, the complete disruption of normal magnetic and gravimetric fields, time warp distortion, and radiation variations. Magnetic fields nullified, dilithium crystals were quickly drained of power, and mass attained zero gravity. This event occurred when anti-Lazarus stepped through in 2267. (TOS: "The Alternative Factor")
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 stopped it. Space was white and full of black stars. Although the universes had totally different timelines, the planets and stars of the Milky Way Galaxy were at least 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 was possible to use molecular structure records in the transporter system to restore the physical age though. (TAS: "The Counter-Clock Incident")