What's special about the Mirror Universe?

Question

The Mirror Universe is depicted in no fewer than eight Star Trek episodes.

It's established that these take place in the same universe. In "Crossover", Mirror Kira recaps the TOS episode "Mirror, Mirror" and how events played out thereafter:

INTENDANT: Interesting. On my side, Kirk is one the most famous names in our history. Almost a century ago, a Terran starship Captain named James Kirk accidentally exchanged places with his counterpart from your side due to a transporter accident. Our Terrans were barbarians then, but their Empire was strong. While your Kirk was on this side, he met a Vulcan named Spock and somehow had a profound influence on him. Afterwards, Spock rose to Commander in Chief of the Empire by preaching reforms, disarmament, peace. It was quite a remarkable turnabout for his people. Unfortunately for them, when Spock had completed all these reforms, his empire was no longer in any position to defend itself against us.

Yet in "Through The Looking Glass", Sisko seems to think it's just one of many possible parallel universes:

SISKO: If I had to guess, I'd say that this is the same parallel universe that two of my crewmembers visited a year ago.

Furthermore, we saw a huge number of parallel universes in TNG's "Parallels". At the climax, we see:

DATA: Other realties are emerging into our own [...] At this rate, the sector will be completely filled with Enterprises within three days.

And from this question, we know there are at least four different apparently totally unrelated ways to travel between the Prime Universe and the Mirror Universe.

So, if there are so many parallel universes, and the Mirror Universe is apparently one of these nearly infinite number of parallel universes, is it explained what is special about the relationship between the Mirror and Prime Universes that all the different means of travel all connect these two universes in particular?

Answer 1

Never addressed in canon.

Unfortunately, there is no canon information that answers your question. The uniqueness of the mirror universe amongst parallel universes is never addressed in the episodes themselves, nor has this topic been specifically addressed by writers or producers, either in the TOS era or TNG era.

As far as I am aware, it is also not addressed in novels or comics.

What's unique about the mirror universe?

Without official information, all we have to work with are observations about what is unique about the mirror universe, or unique about its relationship to the prime universe.

In the two-part Enterprise episode "A Mirror Darkly", we learn that the USS Defiant from TOS "The Tholian Web", which vanished in that episode from normal space (almost taking Kirk with it), actually rematerialized in the mirror universe — but rather than in the time of Kirk, it appeared in the time of Archer.

In this instance, something was transported not only from the prime universe to the mirror universe, but also between two times, the present in one and the past in another.

Worf's jumps between the various parallel universes seen in TNG "Parallels" were always at the same point in time as in the prime universe.

This suggests that somehow, the relationship between the mirror universe and the prime universe might be different than the relationship between the prime universe and those parallel universes.

Answer 2

The word "parallel" is used two different ways.

The divergent universes of "Parallels" are parallel in a pretty loose sense: They only resemble each other to the extent that they share recent common "ancestors." The universes Worf visits are fairly closely related: They all arose from a point after which Worf had started working on the Enterprise, and in most of them he attended a bat'leth tournament (with varying degrees of success). But the many-worlds theory around which this episode is based states that literally zillions of other universes exist:

DATA: For any event, there is an infinite number of possible outcomes. Our choices determine which outcomes will follow. But there is a theory in quantum physics that all possibilities that can happen, do happen in alternate quantum realities.

This includes the infinity of universes where Worf said "Oh, I guess I'll skip the tournament," the infinity of universes where Worf got killed on an away mission in season 1, and so on. Each of these universes is "parallel" to the others in that they exist at the same time as the others, and have a common ancestor at some point in the infinite tree of infinite branches, but many of them—most of them?—don't resemble "this universe" at all, since there are infinite ways in which they differ. They're really more "alternate" universes than they are "parallel."

The Mirror Universe is parallel in a stronger sense, however. From what we've seen, whenever a nice guy is born in the Normal Universe, a guy with the same name is born in the Mirror Universe, and he is mean instead of nice. Whenever good triumphs in the Normal Universe, evil triumphs in the Mirror Universe. Whatever your facial hair choice is in the Normal Universe, Mirror You's choice is the opposite.

And this has been going on for a while. It's not an issue of a quantum event creating universes that gradually resemble each other less, but of events in two universes eerily matching up, "mirroring" each other if you will, consistently over the course of centuries. Worf's many worlds resembled each other superficially as a matter of proximity, but the Mirror Universe really does parallel the Normal Universe: Two timelines side by side, rather than branching all over the place.

Within a many-worlds framework, it's trivially the case that one (or infinitely many) universes will match our universe in eerie ways. But the prime and mirror universes are connected in a special way, which I think serves as evidence that the Mirror Universe doesn't follow the same many-worlds rules.

It's telling that the many-worlds theory only really applies to "Parallels," and elsewhere in Star Trek the alternate realities arise from "few-worlds" origins like the Mirror Universe (I'd argue), time travel shenanigans, and pranks pulled by Q. Infinite alternate quantum realities are only necessary for the purposes of that one episode.

The quote from Sisko in the question may be the only other intimation that such universes exist, and I'm afraid it comes down to a fiddly matter of semantics:

SISKO: If I had to guess, I'd say that this is the same parallel universe that two of my crewmembers visited a year ago.

Does the word "same" here imply that Sisko is differentiating this parallel universe from infinitely many others? Countably many others? You could even read him as saying "that same parallel universe, the only one we have, as opposed to an illusion or temporal instability."

Whatever Sisko's private thoughts, I think it's clear that the Mirror Universe is a unique kind of parallel universe, not "special," but in a class of its own.