Is there any proof it is becoming less "dense" ?
As space expands, the average "density" of the universe goes down, obviously.
I was talking about the mass of the Universe expanding, which may not even be true.
You said 'space' expands. If it expands, what is it expanding into, and how would you go about measuring that ?
If you and I were to stand back to back for a duel, and then walk away from each other, is "space" expanding between us, or are we just moving away from each other "in space" ? Is the space between us less dense, more dense, or the same ?
Yes. The expansion of the universe is extremely well documented. BTW, the average density of the universe is roughly 5.9 protons per cubic meter. This does not take dark matter or dark energy into account, mind you.
I was talking about the mass of the Universe expanding, which may not even be true
The mass of the universe is not increasing.
You said 'space' expands. If it expands, what is it expanding into, and how would you go about measuring that ?
It's not expanding "into" anything.
https://www.universetoday.com/1455/podcast-what-is-the-universe-expanding-into/