According to the theory of cosmic inflation, the entire universe’s size is at least 10^23 times larger than the size of the observable universe.
That’s a lot of universe that we are missing. So, what *exactly* are we missing? What is outside the observable universe? Unfortunately, since we can’t see it or measure it, we don’t know what lies beyond the bounds of the observable universe. However, we have several theories regarding what exists in the great unknown.
Despite its strangeness, this first idea is one of the easiest to digest. Astronomers think space outside of the observable universe might be an infinite expanse of what we see in the cosmos around us, distributed pretty much the same as it is in the observable universe. This seems logical. After all, it doesn’t make sense that one section of the universe would be different than what we see around us. And honestly, who can envision a universe that has an end—a huge brick wall lurking at its edge?
So, in some ways, infinity makes sense. But “infinity” means that, beyond the observable universe, you won’t just find more planets and stars and other forms of material…you will eventually find every possible thing. Every. Possible. Thing.
That means that, if this holds true and we follow it to its logical conclusion, somewhere out there, there is another person who is identical to you in every possible way, and there is also a you who is only slightly different from you in every possible way (one is an inch shorter; one got hit by a bus 5 years ago and died; one has a missing finger etc.). In fact, this “other you” may be reading this article right now; the only difference is that they just picked their nose while you didn’t (or did you?). This notion seems inconceivable. But then, infinity is rather inconceivable.
Another theory deals with something called “dark flow.” In 2008, astronomers discovered something very strange and unexpected—galactic clusters were all streaming in the same direction at immense speed, over two million miles per hour. One possible cause: Massive structures outside the observable universe exerting gravitational influence. As for the structures themselves, they could be literally anything: Amazingly huge accumulations of matter and energy (on scales we can hardly imagine) or even bizarre warps in space-time that are funneling gravitational forces from other universes. We simply don’t know what these massive objects could be. Notably, recent analyses have claimed to debunk the dark flow model, but this debunking is still being disputed.
Another option involves a universe of universes. Some believe that the whole of our universe could exist in a small “bubble” in the midst of a vast array of other bubbles. Theorists call this a “multiverse.” Interestingly, the idea asserts that these universes can come into contact with one another—gravity can flow between these parallel universes, and when they connect, a Big Bang like the one that created our universe may occur.
These ideas are just a few of the more popular hypotheses. There are a lot more out there but we don't about them right now.