The multiverse theory would suggest the existence of infinite worlds, one which the infinite is caused by the variations of a single event such as a person deciding to take a cold shower while in another the person decides to take a hot shower. This situation would be the condition of a butterfly effect where a minor isolated event can cause a cataclysmic incident one which have no direct connection with the former. Or in another case variations of multiple events in which the two universes are frighteningly different, like for instances of various fictional worlds where the setting is still Earth but not the earth we know of; a cyberpunk, a steampunk, a post apocalyptic, dystopian or utopian worlds.
But the truth is that we don't know if these worlds exists and even if they do the chances of us, we observing or coming into contact with them are highly improbable. But with the existence of various literatures one which we could read and in a sense live in those worlds, it's is without a doubt certainly enjoyable. Who knows maybe there exists an utopian wold somewhere I am happy, or even better one which I don't exist cause why not, existence is pain. But for now, it does not matter. Against everything, the theory of a multiverse, infinite universe does sounds nice.
Back to our universe, there is no greater pieces of fictions that embodies this more than the fictional worlds of Marvel and DC Comics.
The whole multiverse thing in comics began with DC Comics in 1961, the 123rd issue of The Flash. Without unnecessary info dump, comic books have to keep up with times and as such characters also have to age and die and also come back for whatever's sake. You can't have 1950's Superman in 1970's. In order to avoid such scenario, the whole story gets rewritten anew. But the old story still exists and is still canon. So that's why we have things such as Golden Age of Comics ( though they most refer to periods of time), Silver Age, Bronze Age and Modern Age. So the consensus would be that these different version would exists in different universes which is then probably rewritten again.
This is also true for their films and televisions counterparts. The great Justice League Unlimited's Batman meeting Batman Beyond's Batman or the newer three generations of spiderman in films. If course these are just the more popular (mainstream) forms of multiverse which exists.
There is a character in Marvel, well two of them to be exact but one of the two is clearly superior over the later. The former being Wade Wilson's Deadpool and the later being Jennifer Walters She-Hulk. As for the reason of these two character and why, because they have one of the greatest ability; the ability to break the fourth wall. Fictional characters be it from books, films, comics or any form of literatures are inherently on a lower dimensions compared to their creators. Because they are fictional they cannot know of the real wold outside their own much less interact with it. Sometimes authors or writers create characters and stories in such a way that these characters can interact with their readers or something the stories can interchange with the real world. This is called breaking the fourth wall. One of the most infamous is the story of Deadpool, this also includes the famous ' 'Deadpool kills the Marvel Universe' and one of the Fantastic Four storyline where they met Stan Lee. The whole breaking the fourth wall troupe isn't only for comics, it also happens in other literatures.
Going further down the rabbit hole. In modern times, DC Comics wasn't doing well and revenue was down. It was under the threat of bankruptcy and possibly being sold off. During that time it released one of the greatest series call Metal series. Long story short, DC Comics had a wall around their universe/multiverse called the Source Wall. This Source Wall encompasses the entirety of DC Comics as a whole. Outside of the Source Wall is or could be anything; like Marvel Comics or Harry Potter or form of fictional world or perhaps even the real world. The Source Wall had been broken in Metal series which led to terrible consequences and is now leading to the destruction of the multiverse (in reference to DC comics possible being bankrupt and closing). Entities refered to as the hands comes outside of the multiverse would destroy the multiverse that would end DC Comics. Of course that didn't happen and DC Comics is still going on. But this is one of the circumstances where real life circumstances affects fictional worlds.
Now why am I saying all of these. Well there are various theories about our reality; idea of multiverse, higher beings and dimensions, idea of a simulated world, great filter and more. And why not add one more; the idea that we live in a fictional world. As crazy a it sounds it's not that far fetched from the others. If we are living in a fictional world it also means we are simulated in a way, higher beings means our creators and them being higher dimensional creatures.
But wait, that's not all. We aren't just living in a fictional world, we are living in a fan fiction. Well, why not? I mean we have multiple God's and Pantheons all claiming to be the creator so why not say they are all right. To be fair, Gods and Pantheons are specific to their own regions, you don't see the Norse Gods in Asia. Of course for us modern humans, unless you are a religious person these are considered fictional. Unless they are for this scenario which would meant that our supposedly fictional world is created by multiple deities but those deities only created specific regions of the world. And when one deity is having enough another one continues with the job thus apparently resulting in the creation of our supposedly fictional world.
While this is definitely impossible, and the idea of it came out from a person who had a bad acid trip, it can't be anything worse, right?