It is one of the most asking question since basically from beginning of time.
The main question is that, what is outside of boundary of our universe...
Cosmologists till are not sure whether the universe is finite or infinite.
If the universe is geometrically flat then universe then universe can be infinite..
And if the universe is curved like Earth then universe can be finite.
A MATTER OF PERSPECTIVE
When you imagine the universe, you might think of a giant ball that's filled with stars, galaxies and all sorts of interesting astrophysical objects. You may imagine how it looks from the outside, like an astronaut views Earth from a serene orbit above.
But the universe doesn't need that outside perspective in order to exist. The universe simply is.
It is entirely mathematically self-consistent to define a three-dimensional universe without requiring an outside to that universe.
Asking "What's outside the universe?" is like asking "What sound does the color purple make?" It's a nonsense question, because you're trying to combine two unrelated concepts.
It could very well be that our universe does indeed have an "outside." But again, this doesn't have to be the case. There's nothing in mathematics that describes the universe that demands an outside.