As with so much in modern physics and cosmology, in this case even asking questions raises questions. Asking what’s outside the universe carries with it any number of implications and assumptions, each of which have to be addressed and, ultimately, deflected with conditional language. However, there’s no better or more convenient way to phrase the question, so we’ll go forward with it: What is outside the universe?
There is only one group of scientists who can really claim to know, and these are the people who reject any definition of the word “universe” which would allow that question to make any sense. To them, the term is interchangeable with “reality,” and anything defined as “outside” the universe would be a part of the universe by virtue of the simple fact that we’ve defined it as existing at all. Increasingly, though, the word universe is acquiring a more meaningful definition that we can work with productively.
One idea of the universe says that it is finite but never-ending. To understand this, imagine a telescope with an infinite ability to zoom in, aimed perfectly away from the center of the Earth. If there was no obstruction to get in the way, zooming in far enough in one direction would under this model let you see the other side of the Earth. That’s assuming that the Earth was there some 50 billion years ago when the light would have left to enter your telescope, which is unlikely since that’s several times the current age of the universe.
The point of the thought experiment is to impress the idea of a universe that is curved around a fourth dimension we cannot directly perceive. In this conception there is nothing outside the universe because there is no edge to be beyond — at least, not one that can be intrinsically grokked by the human mind.
Even if we confine ourselves to the idea that the currently observable universe is simply the area over which matter from the Big Bang has thus far traveled, we come up against a problem: what is this matter expanding into? Under this conception of the question, we must grapple with the idea that the universe doesn’t necessarily need anything to expand into. Space itself has been famously defined as “No more and no less than what is measured by a ruler.” In other words, though we tend to talk about space-time as a substance like a rubber sheet, it is not a thing that has physical reality. It is therefor perfectly possible that beyond the universe of matter there is quite simply nothing. But who’s to say that the nothing isn’t part of the universe, too?
And then the theories start to get really weird. If the universe is infinite, it would also contain an infinite amount of matter. In this case, literally every possible arrangement of matter is present an infinite number of times. There are an infinite number of Earths out there, if we look far enough afield, some drastically different from outs, some virtually identical, some literally identical. Actually, there would be an infinite number of every one of the infinite possible Earths. As to what’s outside this universe, well, there’s obviously nothing beyond an infinite border.
Then there are the multiverse explanations. These postulate that the universe split off after the Big Bang into everything from bubbles to sheets. Our universe is just one of many, possibly a finite number or possibly infinite. In this conception, what’s “outside” our universe is simply another universe. It could have identical physical laws to our own home, or have completely different ones. Everything from gravity to the strong nuclear force could be different, leading to a reality that could behave differently in fundamental ways. Perhaps elsewhere in the multiverse, all matter condensed down into a ball again and is gearing up for a new big bang. Black holes are often cited by cosmologists as possible tunnels to, or even creators of, other universes. They are areas where the laws of physics break down, and where anything is possible.
The multiverse idea actually provides a useful definition for universe: perhaps a universe is the total amount of matter, energy, time, and space that is all interacting and subject to the same fundamental physical laws. Anything that does not interact and conforms to different laws would be “outside” our universe — though of course “inside” its own.
Ultimately, there is only one real answer: nothing is beyond the universe. Do not invert the grammar here; it would not be accurate to say that beyond the universe is nothing. Phrased in the first way, this sentence can mean either that the extent of the observable universe is all there is and there is a theoretical nothingness beyond it, or that the universe is such that it has no exterior and there is no such thing as outside it. Only the multiverse theory has a concrete idea of what might lie outside the universe — but even that is just more universe.
At present, cosmology has a fair bit of evidence suggesting that what we know most conventionally as the observable universe isn’t all there is to be found. Though the idea of “dark flow” as proof of outside universes has been debunked, the concepts of dark matter and dark energy present frameworks for a plane of reality totally beyond our powers of observation. When it comes to finding the edges of the universe, we’ve really only just begun.