That was the "big picture" I was trying to get at, looking at the universe as a single, unified system. At this level of observation, it looks to me like it's a closed system, for the reasons you give, dearest sister in Christ.
At this level of description, running imaginatively backward in time, eventually we have to grapple with the "ex nihilo" condition ... by virtue of which there are no sinks or sources that the universe as a material system can be open to.
Unless its Source is God.
But that's not a scientific statement and thus is not credible with some of our correspondents.
Thank you so very much for writing, dearest sister in Christ!
Truly, space/time does not pre-exist but is created as the universe expands. There was a beginning of real space and real time.
No physical cosmology theology obviates creation ex nihilo - and only Tegmark's Level IV cosmology is closed.