Marvel, DC, they all have them. No, not super heroes dressed in tights with pecs bigger than the chests of most women ( We wonder why men like looking at other men with cleavage, though perhaps we'd best leave it at that). Rather, we explore the concept of parallel universes. There's the 52 comic in DC, and the spiderman multiverse in Marvel just to name two.
Why do they create these far-out otherworlds? Well, the most common reason would be to explore what-if scenarios such as "what-if Lex Luthor really does kill Superman" without the nitty gritty problem of the character actually dying or changing in the main comic theme. This allows them to revive interest on the one hand while not changing the comic to the extent that readers can no longer recognise the characters. Its a win-win really, though it's also a really cheap trick.
Ok, heres the science behind it all. There are many views on the possibile explainations of parallel universes, one such begins at The Begining: the big bang theory. The universe was not always in existance and in fact exploded into being from a point singularity. Scientists speculate that all that energy and mass could have been channeled from another universe, hence black holes in our universe potentially lead to "light holes" and the creation of children universes. Another theory starts at the collapse of a universe, as gravity pulls all mass and space together into the size of a....for fun's sake lets say pineapple. This eventually becomes unstable and explodes into children universe. Thus theoretically infinite universes exist that die, unfold in an eternal symphony. Some have entirely different physical properties, some similar, possibly so similar that we might well have an almost exact replica of earth.
Another theory plays on our very perception. As people we experience three dimensions, excluding time which is not a spatial dimension, forward-backward, sideways, upward-downward. we can experience these three dimensions and anything within them. Lets suppose we live on a piece of paper, 2 dimensions are our world. We perceive Foreward-backward and up-down, but not sideways through the paper. Thus a 2d man would not be able to see outsied of the page, as he cannot perceive it. Another page is place next to his but he does not perceive that page. Suppose that 3d is not all we have. if there were 4d, sideways,up-down, foreward-backward aaannndd.....banana, we would not be able to observe another 3d world placed parallel to ours in the direction of the 4th dimension-in this case banana.
Lets say you punch a hole in 2d man's universe with a pencil. He does not perceive the pencil but thanks you (you are "god" in this case-just this case so do not go poking people with pencils)for the nice ball. If such higher dimensions do exist, the very objects we use and see could be verry different....a simple papaya could be a higher-dimension's alien booger because only the papaya shape is all we could perceieve. Thats not likely but it as possible as most far out fantasy book claims. Anyway I hate that fruit so it may as well be alien booger.
Do note that actual physics theory is far more complex and involves strings, membranes, bosons and a number of complex mathematical equations.