As unlikely as it is, I like to imagine our universe being shaped like a hypersphere.
So for those who don't know, hypersphere is just a sphere in 4 dimensions (w, x, y, and z). But how do you visualize a hypersphere with our measly 3 dimensional brain? Let me show you.
Let's start with a simple flat plane where there is only x and y axis. You can go forwards/backwards and left/right. Simple.
Like an ant on a flat piece of paper. The ant can only travel in 2 dimensions. If the ant starts moving straight in one direction, it will eventually reach the edge of the paper (if the paper is finite) or reach infinity (if the paper is infinitely large).
But what if the piece of paper is curved on the surface of a ball?
Well pfft, that's dumb because then it's no longer a flat 2D plane and becomes a 3D plane. For you. As far as the ant is concerned, it is still moving on a flat 2D plane. It can only go forwards/backwards and left/right.
So if the ant starts moving in one direction, maintaining a straight trajectory and never turning, it will eventually end up where it started.
For us in the 3 dimensional world, that's just obvious. But the ant doesn't know it's living on a curved plane. It is stuck in 2 dimensions.
This is like us on the surface of the Earth. On short distances, we don't see the curvature of Earth. But we know that if we pick a direction on the surface of Earth and starts moving that way, eventually we will return to where we started.
On your backyard, you need to run in a circle to get back to where you started. But this motion requires turning. This motion requires you to follow a curved path. But you can achieve the same if space itself is curved. You don't need to turn, the curved space will do it for you.
If we scale this up to 3 dimensions, we can get the same experience. Theoretically, if our universe is a hypersphere, we can start moving in one of the 3 dimensions and eventually end up back where we started. We're like the ant on the surface of the ball. But instead of 2 dimensions, we're stuck in 3 dimensions.
A 4-dimensional human observing our movement on a hypersphere wouldn't be confused because it makes sense to them.
As it is we currently don't have a strong evidence that our universe is a hypersphere. It is a possibility, but a very very small possibility. Observations suggests that our universe is flat, or has an extremely unnoticeable curvature. We don't know which one.