My question is more like "to what extent does perceiving or not perceiving something determine whether and to what degree it can be said to exist?".

I recommend looking through the Quantum Physics Sequence, or at least the "Quantum Physics Revealed As Non-Mysterious" and/or "And the Winner is... Many-Worlds!" subsequences. Aside from the general matters of map versus territory, our specific knowledge of quantum physics indicates that the observer/collapse effects you may have heard about are not part of what the universe is really doing.

As for self-perception... for all the problems with Descartes's philosophy of mind, "cogito ergo sum" is still a pretty good standard, at least for setting a bare minimum baseline for a definition of existence. (That is, if your definition of existence doesn't allow you to be pretty confident that you yourself exist, it can't be a very good definition.) Further, based on the assumptions of materialism and reductionism (see Zombies? Zombies! and GAZP), I concluded that if a being (whether a normal human you're interacting with, an AI, a person in a simulated universe, etc.) says that they feel conscious, real, etc., and you are confident that they have some mechanism for actually acquiring such a belief that is at least as good as your own (e.g. their program has to actually be mindlike, not just printf("I experience qualia! How mysterious!"); exit(0)), then you should take their word for it.