In fact, there is nothing outside the universe, which is really difficult for people to understand! The universe began with a big bang 15 billion years ago! The universe is still expanding today! There is nothing outside the universe, that is, there is no space and no time. The universe is finite and boundless, just like the earth is finite and boundless. If you walk in one direction on the earth, you will never fall and you will eventually return to where you started! It’s just that the earth is 2-dimensional, and the universe is 3-dimensional! No matter which direction you go from the earth, you will eventually return to the earth! (Of course, this requires you to move very fast, otherwise the universe will shrink to a singularity before you return to the earth)

Reference: Source: Hawking's "A Brief History of Time"