This is sort of a chicken-egg conundrum or asking if a tree truly falls if no one's there to confirm it, so there may not be an answer, but here goes... Does gravity exist if the universe is devoid of all matter, including the dark-matter? I suppose it might given that we are now talking about the dark-energy, but even that would mean no gravity can be detected for all intents and purposes if the distribution of dark-energy were uniform, wouldn't it? So, does gravity exist only if there are means to detect it (i.e. via something like matter)? Also, without getting too philosophical, does "reality" consist only of things that can be measured (sure seems like it)? If so, since the very act of measurement requires having a reference (call it ground state), is everything we can measure all relative to some ground state/value? Finally, assuming that the logic holds thus far, can this reference or the ground-state vary (universally) and still produce the same measurement as before? I guess what my questions are leading to is whether or not there can be a single Universal Constant that varies, but is tied to other Constants in such a way that relative values being measured (with the Universal Constant) are consistent so that the universe as we know it doesn't break down. I'm probably reiterating things in layman's terms that have already been addressed technically elsewhere, so I apologize if this post is a repeat or too trivial. I also make this post because I find it fascinating that nature inevitably give rise to many chicken-or-the-egg scenarios where existence of one depends on the other without leaving little or no clue as to how either one initially got started.