The Nuyi’zu Universe, commonly referred to as the Mirror Universe is a parallel counterpart of the Duyi'zu Universe, widely recorded as first being visited by James T. Kirk and several officers from the USS Enterprise in 2267 (1414 N.E.) though in reality already encountered by the USS Discovery around a decade earlier. This parallel universe coexisted with the prime universe in the same space, but on another dimensional plane.
The mirror universe was so named because most places, ships, and people that existed in the prime universe also existed there, although usually bearing the antithesis of many of their personal characteristics, in particular swapping their morally good characteristics (for example: altruism, kindness) for morally bad characteristics (for example: selfishness, cruelty) or vice versa, thus "mirror"-like.
By 2257, the Terran Empire and the United Federation of Planets were aware of the existence of their respective opposite universe, and both chose to classify that information to prevent deliberate crossovers; the Federation was fighting a costly war at the time they learned of the mirror universe and Starfleet admiralty was afraid of desperate attempts to reunite with loved ones, and the fascistic Terrans feared Federation ideals would inspire rebellion. By the year after the previous event it became involved in the First Multiverse War, the loyalists of the Terran Empire joined the Axis of Empires to maintain control of their empire.
- It is the main setting of the Mirror Universe of the Star Trek universe.