The Multiverse Theory
The amount of space in our universe is infinite. In saying so we can almost assume that there are more galaxies and universes out there. We arent the only universe, we are what we can call the only observable universe, which is what most people mean when they say universe, Because there is not only one. Meaning that out there in space there could be an infinite number of parallel universes.
Now imagine another earth where everything is exactly the same as ours. Another you exists, that has made the exact same choices as you have and has the same life as you. It’s the same for the entire universe, it’s an exact duplicate. This is called a level 1 parallel universe. It is unreachable, it is past our Hubble volume, meaning it is past our reach in sight and in communication. It is farther away than the speed of light has reached. (That’s very far considering light has been moving for 14 billion years) This level is categorized as being the bubble model.
In a level 2 parallel universe the laws of physics could be different from ours, the laws would still exist just within different parameters. In a sense the universe would exist differently than ours but be fundamentally the same. These universes can be called the membrane (or “brane” for short), model.
A level 3 parallel universe is the one you hear about most often in science fiction stories, where events play out in every possible way. So ultimately a completely upside down universe exists, you could look and be the exact opposite of who you are right now. Every choice you could have made in a situation exists somewhere on some universe. This is called the many worlds model.
Here’s a video that will help: The True Science of Parallel Universes
Another link I found extremely helpful: What Are The Types Of Parallel Universes
This is the Multiverse Theory or as I learned, a part of String Theory.