This doesn't change the fact that you are claiming that something is a scientific consensus based solely on an uncited line from a wikipedia article. That is my problem, sorry if I've not been clear about this in my previous posts.
I would argue that some things are so obvious that you really don't need to find a citation for them, and that this isn't a problem. For example, I wouldn't look for a citation to prove that you'll find some H
2O in a glassful of orange juice. It's a true statement, but honestly I doubt a citation for it exists because of how oddly specific and simultaneously trivial the issue is.
When presented with a Quora post (one person's opinion, any single person could have posted it), I provided something better - an article that stood unaltered (until your intervention) for quite some time, having been reviewed by a number of users. Is it the be-all-end-all of scientific citation? No. Is it a damn sight better than what my opponent had to offer? Firmly so.
But none of that matters, because I provided supplementary reasoning and explained my logic. It is impossible for the Universe to be an isolated system, because it has nothing to be isolated
from. The claim that the Universe is or isn't an isolated system is meaningless, because it does not have a meaningful definition. If one were to dispute this claim, they should simply provide that definition (which, you'll note, is all I initially asked for, and which I was refused).
In fact, this is the pertinent issue that killed this discussion. We cannot proceed without agreeing on a meaningful definition of what he meant. He's refusing to provide that definition, and I won't simply assume it, because then we'd risk discussing entirely different concepts while using the same words. Even if it did turn out that I'm mistaken about the consensus, we can't proceed until he fixes his argument.