The acrobat and the tightrope
Mike, Dustin, and Lucas eventually prod their science teacher, Mr. Clarke, for a possible explanation of the Upside Down. He uses a simple, but confusing, analogy: He tells them to picture our dimension as a tightrope and the people in it as acrobats. We're able to walk forward and backyard across the rope, but we cannot turn upside on it without losing balance and falling. He then tells them to picture a flea on the same tightrope. The flea isn't bound by the same gravitational properties because of its physical makeup: it's smaller, more agile, and go all around the rope. The flea, per this metaphor, can access "the Upside Down.”
That's not a super great explanation, considering both a human and flea would occupy the same three-dimensional world. But, in an interview with Business Insider, physicist Paul Steinhardt used a better analogy. Instead of a tightrope, he suggested picturing a sandwich, with each dimension represented as a piece of bread, separated by a layer of hummus. The hummus is the binding between these worlds that makes it difficult for the two to converge; it would require a great deal of energy – like a large finger pressing the two layers together, creating a hole – for them to combine and occupy the same space. In this analogy, Eleven is the finger; she uses her bounty of telekinetic energy to merge the universes together until they create a hole, allowing the physical beings on either side to cross between both planes.
Who knows. Perhaps it always existed, or perhaps it was created by other Hawkins National Laboratory experiments. In reality, there are multiple theories about the possible existence of other universes, and none really come close to explaining the Upside Down. Is it part of a multiverse, one of an infinite number of other worlds? Or is it merely a pocket universe, a projection of our own observational plane? We're not given enough information to summon a guess. If we had to, we'd say it's the latter, and that Season 2 will go deeper into the mythology of the Upside Down's creation.