Yeah this is what I don't understand. In order for this not to be true, space would have to be curved back in on itself like the surface of a balloon. Meaning y

Yeah this is what I don't understand. In order for this not to be true, space would have to be curved back in on itself like the surface of a balloon. Meaning you could eventually wind up back where you started. But do we know for sure this is how the 3D space in our universe works?