Multiverse theory

[MentalGiant]

Simply put-
Space is infinite, the universe is not. The universe is a finite "structure" within space, often visualized as a bubble. Of course, we are currently only able to see a portion of our universe even with the most advanced technology (called the visible universe). The universe is much larger than we can see, and may ever be able to see. Then again, a hundred years ago the concept of other galaxies outside our own were not yet known, now we know about galactic clusters and billions of individual galaxies. Technology is moving so rapidly, things we may not see feasible could be common knowledge in a relatively short period of time.

As far as I am aware, in the theory of eternal inflation, there is nothing like a barrier to actually stop us from leaving our own universe and venturing into "open space" (in theory), similar to how there is no barrier to stop us leaving earth except our own technological limitations. Our current technology limits us from even getting close to leaving our universe though, we're just barely starting to venture off our own planet into our own solar system. Hypothetically speaking, if we DID leave our universe, there would of course be no guarantee (in fact it would be highly improbable) we would be able to exist in other universes, as the set of circumstances which set them into existence would be so vastly different than out own. They may be dictated by such fundamentally different laws of reality we wouldn't be able to travel to them. The really mind blowing shit about it is we may be part of a multiverse that is part of a multiverse-cluster that is part of an even bigger structure (similar to how our universe is made up of galactic superclusters that are made up of galactic clusters that are made up of galaxies that are made up of individual stars, some of which have local solar systems that might contain planets such as our own).

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Here's a fairly simple visual representation: