This article on Space.com includes five theories that explore the possibility of universes that exist in addition to our own, also known as a multiverse.
Scientists assume that space-time is flat and stretches out infinitely. If space-time continues to expand forever, then it must start repeating at some point, because particles can only be arranged in a finite number of ways in space and time. This is known as the infinite universes theory.
According to this theory, if you looked far enough there would be another version of you - in fact, infinite versions of you. Some of these twins will be doing what you are doing now, while others will have eaten a different breakfast this morning, and others will have made drastically different life choices.
The theory of bubble universes comes out of a theory called eternal inflation, which "suggests that some pockets of space stop inflating, while other regions continue to inflate, thus giving rise to many isolated 'bubble universes.'"
Inflation has ended in our universe, giving rise to stars and galaxies, which means that we might exist in our own small bubble in a vast sea of space. The laws of physics and fundamental constants might be different in other bubble universes compared to our own, which could potentially make for strange qualities or behavior.
The theory of parallel universes was developed from string theory and is based on the concept of "braneworlds" - parallel universes just out of reach of our own. There is a possibility of unknown dimensions that are supplemental to the three of space and one of time that we know. This theory can be developed further to include universes that are within reach of one another, which may collide with one another, causing repeated Big Bangs that reset the universes over and over again.
The theory of daughter universes comes out of quantum mechanics, which covers the realm of subatomic particles and describes the world in terms of probabilities, rather than absolute outcomes. This theory explains that if an outcome has two possibilities, the present universe creates two daughter universes - one for each result.
The theory of mathematical universes is based on the idea that math is the fundamental reality. This would mean that our observations of the universe are imperfect perceptions of its true mathematical nature. According to Max Tegmark of MIT, "A mathematical structure is something that you can describe in a way that's completely independent of human baggage."
Although these theories differ in many respects, each points to the possibility that our seemingly boundless universe is simply part of a larger multiverse.
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