The Mirror Universe is a parallel universe which was first recorded and visited by James T. Kirk and several officers from the USS Enterprise in the year 2267. This parallel universe coexists with our universe on another dimensional plane. The Mirror Universe was named because many people and places seemed to be the exact opposites of their normal selves in our universe, but with numerous good aspects who became evil, and vice versa. The Mirror Universe started appearing in the episode called, "Mirror, Mirror", from Star Trek: The Original Series.
Besides the Mirror Universe, there are also other universes that exist on the other spectrum. These universes may also refer to as "Myriad Universes", where any possible outcome can happen in a different reality. These universes could be a result of time travel when someone goes back in time and change history. It could also be a separate universe where the lives of Star Trek characters turned out differently according to their actions and history. For example, Worf finds himself in different alternate realities in the Star Trek: The Next Generation episode called, "Parallels." In this episode, Worf's life and the rest of the Starfleet became slight different due to their actions in current time and history.
The Mirror Universe appears to be somehow interconnected with the "standard universe" to the point of almost everyone from the original universe having a counterpart as the Mirror Universe. This is despite the wildly divergent histories of the two universes, which would normally render the probability of the same individuals existing and being born on the same dates in the same time.
In the mirror universe, known history is dominated by the Terran Empire. It is not clear when the Empire began. Commander Jonathan Archer once stated that the Empire had existed for "centuries" as of 2155. Archer did not mention how many centuries, but by his statement, the Empire can be traced back to at least 1955, suggesting that it was a Terran political unit before it became an interstellar empire.
First Contact between Vulcans and the Terran Empire took place in 2063, as it did in our universe. The long history of nations warring with each other, the stronger overcoming the weaker, led the people of Earth to believe that conquest was the only means of surviving in the universe. Peace was only a ruse used to determine an enemy's weaknesses and to enable the one offering peace a chance to conquer from within. So, when the Vulcans landed and made their peaceful introduction, Zefram Cochrane shot the first Vulcan to step onto Terran soil instead of welcoming them with open arms (as in "our" universe), and the Terrans, interpreting the landing as prelude to an invasion, raided the Vulcan ship. The shotgun used by Zefram Cochrane later came into the possession of Jonathan Archer, who wondered what would have happened had Cochrane not "turned the tables" on the Vulcans' "invasion force." With advanced Vulcan technology at their disposal, the Terran Empire expanded and conquered other races, including the Vulcans, Andorians, Tellarites, Orions and Denobulans.
In 2155, the ISS Enterprise, under the command of Captain Maximilian Forrest, was the flagship of the Terran Empire's Starfleet. In January of that year, Commander Jonathan Archer mutinied against Forrest in order to take the ship into Tholian space to capture the USS Defiant, which had traveled through a spatial interphase from the year 2268 in our universe. While the mutiny was ultimately unsuccessful, Captain Forrest had no choice but to continue the mission, since the ship's helm had been locked on auto-pilot. The Enterprise was later destroyed by Tholian ships, but not before Archer was able to take control of the Defiant. Archer tried to use the Defiant in a grab for power, intending to replace the Emperor, but was betrayed and killed by Hoshi Sato, who declared herself Empress. However, it remains unclear as to whether Sato actually established herself as Empress, or if the Defiant played any further role in the mirror universe.
Sometime between 2155 and 2267, the symbol of the Empire appears to have been altered. The earlier symbol closely resembles that of the United Earth government, depicting all of Earth's continents, though replacing a laurel of peace with an aggressive sword. However, by the mid-23rd century, the symbol, while remaining essentially the same, depicted only the continents of Earth's western hemisphere.
By 2267, the Terran Empire was the dominant power in the Alpha Quadrant. When four Starfleet officers (Captain James T. Kirk, Lieutenant Uhura, Lieutenant Commander Montgomery Scott and Doctor Leonard McCoy) from the USS Enterprise were exchanged with those same officers from the ISS Enterprise in the mirror universe because of transporter interference from an ion storm, they discovered a brutal regime, almost dictatorial in its command structure. The advancement through assassination was a commonplace in the Mirror Universe.
During the encounter, Captain Kirk convinced the counterpart of his first officer, Spock, that the Empire could not sustain itself any longer. Indeed, Spock predicted that in its current form, the Empire had 240 years before its total collapse.
Shortly thereafter, Spock rose to become leader of the Terran Empire, proposing a series of reforms designed to make the Empire more secure and less dictatorial in nature. These included a significant disarmament program. Unfortunately, once these reforms were complete, the Empire was unable to defend itself against the equally aggressive and powerful forces surrounding it. The Klingon-Cardassian Alliance overran the Empire, conquering Earth and leaving Terrans and Vulcans enslaved, and freeing several worlds that still remained under Terran occupation, including Bajor.
With the fall of the Terran Empire, the Klingon-Cardassian Alliance became the dominant power in the Mirror universe. Bajor soon joined the Alliance, and the command post and ore processing facility Terok Nor was constructed in orbit.
Also during this time, the forces of the mirror universe began implementing safeguards to prevent another crossover event. Transporter design was altered to prevent interdimensional travel, requiring the creation of devices specifically for that purpose, including the multidimensional transporter. In the event of another crossover, those involved would be killed to prevent further interference.
By 2370, Terok Nor was commanded by Intendant Kira Nerys, with Elim Garak as her second-in-command. It was at this point in time that the second known contact with our universe took place. A runabout from station Deep Space 9 entered the mirror universe following an incident in the Bajoran wormhole. Kira Nerys and Julian Bashir were captured by forces from Terok Nor, and interrogated by the intendant. Bashir was sent to work in the ore processing plant, where he befriended "Smiley" O'Brien. After instigating a series of incidents aboard the station, including the death of Odo during a slave uprising, Kira and Bashir convinced privateer Benjamin Sisko to rebel against the Alliance and help them to escape back to "our" universe.
A year later, Smiley crossed over to the Federation's universe and impersonated his counterpart long enough to capture our Sisko, and brought him back to the mirror universe, where he convinced Sisko to impersonate the leader of the Terran Rebellion. The mirror Sisko had been killed in a skirmish with Alliance ships, and the rebels needed the other Sisko to win over Jennifer Sisko, Sisko's wife, who in the mirror universe was a scientist working for the Alliance. Jennifer had been developing a transpectral sensor array, which would have allowed the Alliance to locate rebel hideouts in the Badlands. Sisko convinced his wife's mirror-counterpart to defect to the side of the rebels.
While in the Federation's universe, Smiley downloaded information from Deep Space 9's computers, including the plans for the USS Defiant. In 2372 the rebels constructed their own version of the Defiant, but had trouble getting it to function properly. Jennifer lured Sisko back into the mirror universe to help, though Kira Nerys killed her a short time thereafter. By this time the rebellion had grown in strength, culminating in the capture of Terok Nor, which became a rebel base of operations.
In 2374, Intendant Kira sent a thief, Bareil Antos, to "our" universe to steal one of the Bajoran orbs, believing it would permit Kira to unite Bajor under her rule. The attempt failed, and Bareil returned to the mirror universe without the orb.
In 2375, Grand Nagus Zek, the leader of the Ferengi Alliance in our universe, used the multidimensional transporter to travel to the mirror universe along with his Hupyrian manservant, Maihar'du, hoping to open up business opportunities, but they were captured and held hostage by the Alliance. Kira made arrangements with Worf, the regent of the Alliance, to obtain our universe's version of the Klingon cloaking device in exchange for Zek's return.
She sent Ezri Tigan, a Trill mercenary and Kira's lover, to our side to give Quark the ransom demand. Quark and his brother Rom stole the cloaking device from Klingon General Martok's ship and delivered it to Ezri, but decided at the last minute that they couldn't trust her to keep her side of the deal, and accompanied her to the mirror universe, where all three were captured by the Terran rebels, who planned to keep the cloaking device, until Ezri's companion Brunt freed them, delivering them to Regent Worf. Aboard Worf's flagship, Quark and Rom were imprisoned along with Zek upon discovering Kira's plan, only to be later rescued by Ezri as revenge against Kira for killing Brunt. Ezri ended up joining the rebel cause. Quark, Rom, Zek and Maihar'du were allowed to return to the primary universe for having aided the rebels' defeat of Worf, a major victory for the rebellion. During the escape from Worf's ship, Garak was killed. When last seen, the rebels' march towards victory showed no apparent signs of slowing.
These Star Trek episodes have been taken place in the Mirror Universe.
Star Trek: Enterprise
In A Mirror Darkly, Part 1
In an alternate universe, an amoral and imperialistic version of the Enterprise crew, led by Captain Forrest with Archer as his treacherous first officer, makes an astonishing discovery during a mission. Phlox tortures a captured Tholian for information about the discovery.
In A Mirror Darkly, Part 2
The Mirror Archer commandeers the 23rd-century Defiant from the Tholians and uses it in a nefarious power grab.
Star Trek: The Original Series
During an ion storm, Kirk, McCoy, Scotty and Uhura are transported to a parallel dimension on an alternate, savage Enterprise. In this universe, the Federation is as feared as the Klingons. Kirk tries to convince Spock to see the logic in changing the order.
The Tholian Web
While exploring the remnants of the USS Defiant, Kirk is accidentally trapped in another dimension. Most of the crew believe him to be dead, though they continue to see his apparition. Meanwhile, Spock works on trying to retrieve him when the Defiant enters our dimension again. The USS Defiant later disappears and reappears in the past where the Mirror Archer and his Enterprise crew found it.
Star Trek: Deep Space Nine
A mishap in the wormhole sends Kira and Bashir into the Mirror Universe where Bajor is a tyrannical power and humans are slaves.
Through The Looking Glass
Commander Sisko must assume the role of his dead counterpart in order to save the mirror version of his late wife.
Sisko follows his son into the Mirror Universe after Jake is lured there by the living counterpart of his late mother.
When the Mirror Bariel enters our universe and attacks Kira, she doesn't charge him and tries to fall in love with him again.
The Emperor's New Cloak
Quark and Rom risk their lives by crossing into the Mirror Universe to rescue Grand Nagus Zek whose been captured by the Alliance.