Mirror universe is the unofficial designation for the parallel universe the Terran Empire and the Klingon-Cardassian Alliance originate from. In contrast to the "prime" universe, the course of history has taken a different turn early in Earth's history (and possibly other planets as well). Humanity has not risen to be a respected member in an interstellar alliance of equals, but a totalitarian regime oppressing other races by brutal force.
No date of divergence between the Prime timeline and the Mirror universe has been established, although it is likely it dates back to the 16th century, if there is one, with the Terran Empire dating back to at least the 20th century.
Humanity's First Contact with an alien race still took place with the Vulcans in 2063, although in this timeline Zefram Cochrane shot the Vulcans and the humans seized the Vulcan ship and its technology. The Terran empire rapidly spread across the stars, conquering Vulcan, Andoria, Tellar Prime, Bajor Denobula, and Gorlan by the 22nd century.
Humanity's conquered races began rebelling against the Empire, but its rule was saved by the displaced U.S.S. Defiant, from the Prime Universe's 23rd century. Using the advanced technology from the future, the Empire was able to conquer all of known space, including rendering Qo'noS uninhabitable.
In 2256, the U.S.S. Discovery and I.S.S. Discovery swapped places, although the I.S.S. Discovery (along with a small Klingon fleet) was also displaced in time, sending it to 2410. It was returned to 2256, in the prime universe, shortly before it was destroyed by Klingons; however her captain remained at large in 2410.
In 2267, a transporter accident swapped James T. Kirk with his counterpart. The influence of the Prime Universe Kirk on the Mirror Universe Spock led to Spock taking control of the Empire and reforming it to peaceful and benevolent. This however led to a weakening of the empire and allowed the Klingon-Cardassian Alliance to rise to challenge it. The Klingon-Cardassian Alliance eventually conquers the entire Empire by the mid-24th century with the former Terran rulers reduced to slaves. Bajor later joins the Alliance.
Over the 2370s, people from the prime universe make a number of "crossovers" between the realities and begin a chain of events that lead to the Terran slaves of Terok Nor rebelling against the Alliance and re-take Terok Nor. The Rebellion manages to steal the plans for the U.S.S. Defiant and construct the I.S.S. Defiant. Combined with a stolen Klingon cloaking device, the Terrans manage to fight off Alliance forces.
By the 25th century, the Alliance has fallen to a newly reformed Terran Empire. It is as totalitarian as before and, while it allows non-Terrans such as Leeta to serve in its ranks, it still maintains a highly xenophobic attitude towards them and outsiders. It also begins incursions into the Prime Universe, forging an alliance with the True Way. The Orb of Possibilities, taken from the mirror universe at an unknown time, is returned to mirror Bajor, aiding the rise of Captain Leeta.
In 2410, the Mirror Universe's history is altered significantly when Admiral Leeta defeats the Iconians 200,000 years in the past and takes challenges the Emperor's power using a combination of future technology from the Temporal Liberation Front and aid from the Pah-wraith.
- “Jabberwocky”: The player enters the mirror universe via a portal in the Badlands to return the Orb of Possibilities.
- “Firewall”: Chasing after Kuumaarke, the player enters the mirror universe through a portal in the Izar System to retrieve the Emperor's data.
- Before the revamp of the Cardassian Struggle mission arc, in the mission “The Other Side”, players encountered the remains of Klingon-Cardassian Alliance led by General B'vat and Captain Ja'rod. They assisted the player in an attack on a Terran installation in the Arawath System
(This mission, along with the other cut Cardassian Struggle Mirror Universe missions, was a player-recreated Foundry mission).