If the universe is all there is, it must be infinite. If the universe were not infinite, then it would be banded about by nothing; but if something is banded about by nothing, it is infinite. So a non-infinite universe is a self-contradiction. Things like the “three torus model” of the universe (pic related) are literally nonsensical, because it is being claimed that the space in the middle of the “donut” does not exist, that it is nothing, and so it should be deleted from the picture entirely. The universe is necessarily infinite.