There are two Multiverse hypotheses that seem plausible to me. One is the String Landscape Model and the other is the Many Worlds model.
The former has (according to Brian Greene) about 10500 possible universes (there are approximately 1080 subatomic particles in the Universe we live in). If you think a little like Max Tegmark that anything that is mathematically possible actually exists somewhere, sometime, somehow, then each of these 10500 possible universes are a solution to the differential equations that are what String Theory or M Theory are made of.
The other (Many Worlds) is speculated to have about 101010000000 possible universes. It's based on the notion that if one were to perform Schrodingers Cat experiment, the timeline will somehow split and there will be one universe with the cat alive and another with the cat dead. Every time an electron has a binary probabilistic "choice" of which way to go (a quantum fluctuation) two universes then exist, one where the electron turns left, the other where the electron turns right. Essentially, every story that can possibly be told occurs in one of those universes. In one, I am President of the United States. In another, I assassinate the President of the United States. In still another, I am the president and I am assassinated. Perhaps you will find a teapot in orbit in one of those universes. And in another an Invisible Pink Unicorn. And in another a Flying Spaghetti Monster.
There are other multiverse hypotheses that have very advanced alien life designing and creating universes. Those seem even sillier than the two I mention above.
The main reason why this Multiverse thing gets some proponents and detractors is because of the Does God Exist debate. One (rather strong, in my opinion) argument for the existence of God is the Teleological Argument sorta classically posited by William Paley. A more modern argument is made by pointing out that several of ca. 26 dimensionless fundamental constants of the Universe have to take on values very close to the values they do take on just so that the Universe can cook up matter, astronomical structures, elemental diversity, and eventually life as we know it. However, using the Weak Anthropic Principle (which is a tautology, so it has to be true, albeit sorta an empty truth) and the notion of the Multiverse, a case can be made using selection bias or survivor bias to explain away any sense of remarkability of the ostensible appearance of fine-tuning.
In order to make the case that it is unremarkable that one has been dealt a Royal Flush one must make the case that millions or billions of poker hands have been dealt and only in the case of a Royal Flush will there be anyone around to notice. All other hands are unlucky enough that no life will appear to notice what hand was dealt. But this selection bias case only makes sense when there is a statistical population of universes to select from. Only in such a universe where fundamental constants and initial conditions come out "just right" will there be any beings that have evolved to the point to be asking these questions of "How the heck were we able to come to be in an otherwise life-unfriendly universe?" It's the question that Fred Hoyle was thinking about when he conceived of his 747 Junkyard tornado notion.
The Multiverse hypothesis has as much evidence supporting it as is there evidence for the existence of God. Really, no more. Whether aliens created our universe or if it's just one of the zillion created by some natural universe creating mechanism or if God created our universe, none of these are falsifiable in the Popperian sense. Your speculation or your faith belief, is as good as mine.