Current physics theory postulates four possible forms of alternate universes. This is my interpretation of the theories:
An infinite number of sub-universes exist inside one giant multiverse (multiple universe). The only reason we have not seen the others is because they are so far away that the light has not had time to reach us, yet. All of the universes duplicate each other, and what happens in one universe happens in all of the universes of the multiverse.
An infinite number of sub-universes exist within a giant multiverse, called the block, and these sub-universes are like soap bubbles floating at random through the air. There is no similarity among the sub-universes. All possible combinations of things exist in one sub-universe or another. Humans exist in one of the sub-universes, but might not exist in any of the others, or some sub-universes might contain humans and others might not. A new universe is created every time two of the sub-universe "soap bubbles" touch each other, but the old sub-universes are not destroyed in the process. Thus, the number of sub-universes continues to grow, but the block holding the sub-universes is infinite in size, so it will never be filled.
An infinite number of sub-universes exist within a giant multiverse, called the block, and these sub-universes are like parallel planes. They never interact in the physical sense. A new universe is created every time there is a choice offered. The old sub-universe continues to exist with one phase of the choice and a new sub-universe is created to accommodate the other possibility. As an example, suppose someone drops a coin onto a flat surface from a sufficient distance so that the coin lands on edge. In one sub-universe, the coin comes to rest on edge and never falls over. Two other sub-universes are created, one in which the coin falls heads up, and another in which the coin falls tails up. This is going on all of the time, so every time you decide whether or not to eat that next cookie, a new sub-universe is created. The possibilities here are mind boggling.
The type 4 universe is harder to describe. There is an infinite number of sub-universes in the block, all existing at the same time and occupying almost the same space. They are separated by one or more of the seven dimensions that we cannot detect. The Type 4 multiverse has 11 dimensions: the X, Y, and Z, plus time directions that we perceive, and seven more which describe the way the sub-universes can exist all at the same time. There is a series of experiments going on trying to detect one of these other dimensions, concentrating on a particle called the graviton, the smallest unit of gravity.
The above is not fiction! It is the simplest explanation for the mathematical equations which describe space and time. One day, humans may know which of the four is the correct form of the solution. Of course, there may be a fifth form which nobody has thought of, yet.
I have posted this at the suggestion of "Jake," a reader of my stories. If any author wants to use any of it, please do so. All I ask is that you drop me a note so that I can see how it came out.