Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, host of Ask a Spaceman and Space Radio, and author of How to Die in Space.
It's one of the most compelling questions you could possibly ask, one that humanity has been asking since basically the beginning of time What's beyond the known limits? What's past the edge of our maps? The ultimate version of this question is, what lies outside the boundary of the universe? The answer is well it is complicated.
To answer the question of what's outside the universe, we first need to define exactly what we mean by universe. If you take it to mean literally all the things that could possibly exist in all of space and time, then there can't be anything outside the universe. Even if you imagine the universe to have some finite size, and you imagine something outside that volume, then whatever is outside also has to be included in the universe.
Even if the universe is a formless, shapeless, nameless void of absolutely nothing, that's still a thing and is counted on the list of "all the things and, hence is by definition, a part of the universe.
If the universe is infinite in size, you don't really need to worry about this conundrum. The universe, being all there is, is infinitely big and has no edge so there's no outside to even talk about.
Oh, sure, there's an outside to our observable patch of the universe. The cosmos is only so old, and light only travels so fast. So, in the history of the universe, we haven't received light from every single galaxy. The current width of the observable universe is about 90 billion light-years. And presumably beyond that boundary there's a bunch of other random stars and galaxies.
But past that? It's hard to tell.
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