Interestingly, parts of this universe are right now becoming unmeasurable, un observable, unfalsifiable to us even now. With dark energy propelling the expansion of space at an increasing rate, far-away regions of space can be moving away from us faster than light. This means they’re no longer part of our observable universe - we’ll never be able to reach them, see light from there, interact with these places in any way. Not the same universe anymore.

Someday living beings in the Milky Way might believe it’s the only galaxy that exists because no others will be observed. If one of them postulates that maybe there are all these other galaxies that are just out of reach - pinched off from our universe - in parallel universes you might say…. if someone says all that, it could also be just as handily dismissed.