Parallel universe
A parallel universe, also called a parallel reality or alternate universe, is a self-contained universe running concurrently with the prime reality. () 

There are different types of parallel universes. Some are separate dimensional planes. Some of these planes are governed by completely different physical laws, while some duplicate the primary universe closely. Some parallel universes exist within subspace, while others exist as completely different quantum realities.

The idea of alternative universes had been around since the 20th century. While working on with the experimental spore drive in the mid-2250s, Captain  began seeing the possibility of exploring them. Later, his ship found itself in a hostile parallel universe, seemingly as the unexpected result of a jump,  though this turned out to be the deliberate manipulation of Lorca, a resident of that parallel universe who had been posing as his prime universe counterpart. ()

Types of parallel universes

Trans-dimensional realms 
A multitude of universes exist in the same physical space but have a different dimensional structure from one another. For this reason they do not normally interact. () By the early 2270s, the existence of these other realities could still not be proven logically, even though many of them were known to exist. () There were several methods of entering parallel dimensional planes; one of them was a portal created by the Guardian of Forever. (; DIS: "Terra Firma, Part 1", "Terra Firma, Part 2") 

 The most frequent contact between the primary universe and a parallel universe on a different dimensional plane involved the so-called mirror universe, with the first reported crossover occurring in 2250s. () Passage between the universes was accomplished using a Tholian artificial interphasic rift, the transporter boosted by a power surge from a magnetic storm or the warp engines, a plasma injector leak during the passage through the Bajoran wormhole, and the multidimensional transporter device, which reconfigured transporters to allow passage without the need for a power boost. In the mirror universe it was believed by the Terran Empire that parallel dimensional planes do not exist, until the Tholians opened the rift in 2155 to the future of the primary universe. (; ; , et al.) 

 Interphasic rifts also contained a parallel universe completely empty and devoid of life between dimensional planes. The rift itself allowed interaction between the dimensional planes of at least the primary and the mirror universe by overlapping them for periods of time. (; )

 The Edo God existed between dimensions. It was neither inside nor outside the spacetime continuum. ()

 The two known antimatter universes had the difference between matter and antimatter reversed in comparison to the primary universe. The minus universe could be reached through a negative magnetic corridor. The reverse universe could be reached by flying into a supernova at high warp. (; )

 A trans-dimensional realm existed where all known life was photonic by nature. Photonic lifeforms had the technology to create trans-dimensional portals to travel between the primary universe and their realm inside subspace sandbars. () 

 A solar quantum level cascade periodically caused a quantum fluctuation in the molecular structure of Meridian and its inhabitants. The fluctuation shifted them to an intersecting dimensional plane where all life and matter existed in non-corporeal form as pure consciousness without aging. () 

 In the Megan universe, magic was part of the natural laws. Megans who traveled out of their universe still maintained the ability to use thoughts to control reality, while Humans entering the Megan universe could use their thoughts to alter reality there, but would not maintain the power after returning. Matter in the primary universe originated from the Megan universe through creation points. Creation points were the active portals between the two universes. ()

 The Sphere-Builders were native to a trans-dimensional realm that had its matter composed in such a way that it could not maintain cohesion in the primary universe. Interspatial parasites from this realm could, however, maintain existence in the primary universe as they existed in a state of interphase and temporal flux. The space inside the Sphere-Builders' trans-dimensional realm was similar to that inside a trans-dimensional disturbance, as such disturbances were created by the Sphere-Builders in an attempt to alter the Delphic Expanse so they could survive in and ultimately invade that region. Ultimately, the invasion failed. ()

 The Prophets, while identifying themselves as being "of Bajor", were a non-corporeal extra-dimensional race known to reside in the realm known as the Celestial Temple. The Prophets had the ability to foresee all the consequences of their actions and did not conceive their existence in terms of linear time. The portal to the primary universe was the artificial wormhole. Humans in this dimension had the ability to relive any past event of their lives in the form of subjective or shared visions. ()

 The Traveler's race, even though native to Tau Alpha C, was also a species native to a different reality and time. Empaths, such as Deanna Troi, could not sense their presence in the prime reality. The Traveler used the energy of thoughts to alter warp fields and could step outside of time and enter other planes of existence. ()

 The nucleogenic lifeforms, known as "spirits of good fortune" by the Ankari, were native to a different dimensional plane and could exist in the normal universe for only short periods of time. Nucleonic lifeforms had the ability to generate interspatial fissures to pass between the realms. () 

 When a second Kes appeared on  through a spatial rift in 2372, Kathryn Janeway believed she had come from a different version of their ship. The Doctor began to wonder where this other Voyager existed, suggesting a parallel universe, another dimension, and a different point in time as a solution. It was later discovered that this other Voyager was a duplicate created by a spatial scission, existing at exactly the same point in space, at exactly the same time, but in a state of spatial flux. () 

 In 2373, Tuvok experienced a breakdown on the bridge of Voyager over a traumatic memory suppressed into the subconscious of an event Tuvok had not experienced. The Doctor surmised it could have been caused by any number of things, such as aliens with telepathic abilities or a brief experience with a parallel reality. After describing these to Tuvok, The Doctor commented "the universe is such a strange place." After initiating a mind meld with Captain Janeway, who was acting as Tuvok's pyllora, they both discovered that his breakdown was in fact not caused by contact with a parallel reality, but rather a memory virus unknowingly transmitted to him by his former crewmate from the , Lieutenant Commander Dimitri Valtane, in 2293 during a battle with the Klingons. () 

 Fluidic space was a dimensional plane where instead of space, the whole dimension was filled with matter in fluid form. Passage between the planes could be accomplished through artificial quantum singularities created using resonant graviton beams. There was also a border between the two realms located somewhere in the Beta Quadrant. ()

 A parallel dimensional realm randomly intersected regions of space on the eighteenth dimensional gradient creating regions of chaotic space. In these overlapping zones the laws of physics were in a state of flux. ()

 The Nexus was a dimensional realm where reality shaped itself to accommodate the desires of anyone who entered it. Entry to the Nexus could be gained through proximity to a temporal flux energy ribbon in the primary universe. The Nexus could be exited by an act of will into any desired time and place in the universe. ()

 Galactic Cluster 3 was the Borg designation for a transmaterial energy plane. According to Seven of Nine, it was beyond Human comprehension. ()

 In 2267, Spock speculated that the flying parasites might be native to a place where the physical laws of the known universe did not apply when they were discovered to be a collection of macroscopic brain cells without any apparent connection to one another, yet still capable of acting as a single brain. It was later discovered that they originated from the Large Magellanic Cloud. ()

 The Q Continuum was the realm in which the limitless dimensions of the Milky Way Galaxy were combined. () The Continuum could be only understood by the . However, the Continuum could be allowed by a Q to manifest itself in any number of ways for other lifeforms to interact with it and perceive it in terms they could comprehend. () The Q Continuum could be entered by non-Q when spatial disruptions originating from the Q Continuum caused matter from the primary universe to be sucked inside. ()

 Dr. Paul Manheim also believed in the existence of infinite dimensions. To prove the theory, he created a method of opening a window to another dimensional plane by creating a temporal disruption in our dimension. In the other dimension, the native life had a completely different existence to the primary universe. Until the window was closed, Manheim's mind existed in both dimensions at the same time. The sight of the other plane was so alien, the words to describe it did not exist in the Human language. ()

 First contact was made in 2376 with entities speculated to be composed of dark matter. They resided in a dark airless parallel dimension filled with lifeforms. They had the ability to transport between the realms. It was stated that the creatures realm did not exist in either space or subspace from the prime universe point of view. ()

Subspace domains

There were also an infinite number of parallel universes within subspace. ()

 The matter stream of a transporter beam was sent across a subspace domain. ()  

 New universes formed into subspace naturally inside subspace interphase pockets. ()

 One known inhabited parallel universe in subspace was a tertiary subspace manifold domain. It was home to the solanogen-based lifeforms, who were capable of generating spatial ruptures to the prime universe using inverted tetryon particles. () 

 Exosia was a subspace realm containing only pure thought and energy. The home galaxy of the Nacenes  and Suspiria was located in this subspace layer. The Nacene were capable of generating subspace ruptures to allow passage between the universes. ()

 Transwarp space was the realm of subspace through which Borg transwarp conduits were formed. (; )

 The Borg neural interlink frequencies, the interactive signals that formed the basis of the Borg collective consciousness, were sent across a subspace domain. ()

 New parallel universes could be also be created into subspace artificially by generating stable warp bubbles. The universe formed according to the thoughts of an individual trapped inside the bubble. The nature of these universes was a spheroid region surrounded by a mass energy field. A stable threshold between the universes manifested itself as a dynamic atmospheric disturbance of great intensity. Recreating the bubble in the prime universe generated a phase link with the warp bubble universe. ()

 The warp bubble process was similar to using a static warp shell to seal its contents out of the universe by surrounding them with an artificial subspace barrier. ()

 Subspace sinkholes sucked matter into a subspace pockets. These subspace pockets could contain entire star systems. ()

 Interfold layers were unstable realms located in some regions of space. These realms were not contained within normal space or subspace, but in the confluence of the two. ()

Quantum realities 

There are an infinite number of alternate quantum realities, one for every possible outcome of any event that occurs. Each reality has its matter resonating on a unique constant quantum signature. Quantum universes were separated by barriers from one another. Although each universe was separate, they had a similar past until the particular diverging event occurred. ()

 In the prime quantum reality, in 2370, Lieutenant Worf was the chief tactical officer and security chief of the . He had won the 2370 bat'leth tournament on Forcas III, and was subsequently wished happy birthday by his friend Counselor Deanna Troi alone. Doctor Beverly Crusher was the chief medical officer of the ship, () and Wesley Crusher was a cadet in Starfleet Academy at the time. ()

 In one alternate quantum reality, Worf and the others held the same ranks and positions. Worf had won the tournament, but was given a surprise birthday party by the senior staff except Captain Jean-Luc Picard. The Argus Array had stopped transmitting data for an unknown reason. () 

 In one alternate quantum reality, Worf and the others held the same ranks and positions. Worf won the tournament and was given a surprise birthday party by the senior staff, including Captain Picard. The Argus Array had stopped transmitting data due to a Cardassian takeover. () 

 In one alternate quantum reality, Worf and the others held the same ranks and positions. Worf came in as ninth in the tournament, after suffering a concussion. He was given a surprise birthday party by the senior staff. The Argus Array had stopped transmitting data due to a simple mechanical failure. () 

 In one alternate quantum reality, Worf and the others held the same ranks and positions. When Worf was given a surprise birthday party by the senior staff, he was given a completely different painting as a gift from Data. ()

 In one alternate quantum reality, Worf held the same rank and position, but was married to Deanna Troi. He didn't attend the tournament due to a malfunction in the ship's main deflector. Kurn attended the tournament in his place. Worf was given a birthday party by the senior staff and the new painting as a gift from Data. Doctor Crusher and Wesley were not on board the Enterprise. Data had blue eyes and Ogawa was the chief medical officer of the ship. The Argus Array was destroyed by the more aggressive Cardassians; Geordi La Forge was killed during the battle. Computer consoles had new configurations and the new warp core installed earlier in 2370 was never replaced. ()
  
 In one alternate quantum reality, Starfleet used different communicators, and Riker was the captain of the Enterprise-D after Picard was killed by the Borg in 2367. Commander Worf was the first officer, married to Counselor Deanna Troi with two children, Eric-Christopher and Shannara while Alexander Rozhenko had never been born. Data had given Worf a painting as a birthday present and still had yellow eyes. Lieutenant Wesley Crusher was serving on board the Enterprise as the security chief and chief tactical officer and Doctor Beverly Crusher was the chief medical officer. The Argus Array was destroyed by the Bajorans who had overthrown the Cardassian Union and were increasingly aggressive towards the Federation prior to 2370. La Forge had been killed in a recent battle. At least one Cardassian was serving on board the Enterprise as a Starfleet officer. ()

 In one alternate quantum reality, by 2370, the Federation had been completely wiped out by the Borg. The Enterprise-D was one of the last remaining ships and had taken heavy damage in the fight against the Borg. The ship was under the command of Captain Riker. Lieutenant Worf was serving at tactical. ()

Alternate realities 
Different versions of a single timeline and alternate time continuums also sometimes coexisted as parallel realities or as smaller self-contained pocket realms.
For more information, see alternate timeline.

Appendices

Background information 
A list of episodes where we see a parallel universe.

External link 
 

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