There are some who believe in what they call multiverses. The concept literally means "multiple universes". The basic idea is exactly what it sounds like. There are more than one universe.
Based on the scientist and theory in question, some of these multiverses either interact with each other on some quantum/elemental/whatever level, or they are completely self-sustaining and not connected in any way.
This here is our first clue that these theories might be flawed. Are they connected or not? Kind of a big detail to disagree on, eh? Though there is one huge gaping flaw in their ideas built right into the base assumption of them.
A lot of the concepts of multiple universes come from an understandable offshoot of the basic "Schrodinger's Cat" concept. In short the experiment envisions a case of atomic uncertainty leading to a cat either living or dying. Since the "atomic uncertainty" idea postulates that an atom can be in two states at once until viewed, the cat, by extension, is considered to also be in two states, both alive and dead, until actually viewed. Well, since both states can occur at once, some want to pretend they actually do occur and each split into their own universe.
As I said. Sheer poppycock. However, we'll kill Schrodinger in another post. For now, be very very quiet. We're hunting multiverses.
Regardless of the scientist or theory, multiverses, alternative universes, parallel dimensions, etc., whatever the en-vogue term of the month is, simply can not exist on a basic problem of definitions. The universe is everything. E-v-e-r-y-t-h-i-n-g. Not a section of some matter, but not others. Not some energy, but only until it shifts states. The simple idea of the universe, once appreciated for what it truly is, literally leaves no room for any multiverse.
While I understand the scientists desire to use these hypothetical conceptualizations to explain their theories, the simple fact is their definitions don't work. Which often means their theories don't either.