I pose myself that question when I was 16 or so. Where does the Universe expands to if there is nothing to nothingness, and after reading Einstein it come to me that perhaps the whole of spacetime is already settled, no actual true motion whatsoever is happening...from there I jumped to Parmenides and realized how actual his point on Being is still central in our search for understanding the world around us..
What kind of motion makes sense to have and what kind it does not ?
This is a central question that I never saw properly addressed neither by cosmologists physicists or even Philosophy of Science.
Motion into nothingness certainly doesn't make any sense, as there is nothi nt to move into. From this simple straight forward conclusion it follows the Universe is either a part of a multiverse where the same question
arises, or that as a whole the Universe has boundaries and must be rational, that is to mean finite.
In this context motion is an on/off "reader" of that which already is throughout the whole of spacetime, and that is where our sense of change comes from.
This finite cyclic Universe must therefore have a pattern where the "end" meets the conditions of the "beginning" and repeats forever...another variation of this idea is that each cycle has slight changes in the conditions that themselves for a meta algorythm where X number of cycles produce all possible worlds within the "Rubik's cube of this discrete X size space that goes back n forward between big bangs...
In sum I am no cosmologist, not even a professional philosopher but I have a very clear idea of what can't be the case. An irrational boundless ever expanding Universe going forward into nothingness doesn't make any sense.