Gringinho, I'm intrigued as to what your point actually is. If Philosophy uses cause-effect and logics to reason about the universe as you say, then it is reasonable in that same philosophy to postulate everything is inescapably dependant upon existence.
"sure, the abstract idea or concept of "nothing" does exist"
..then it depends upon existence to exist. A state or condition or potential or possibility of existence (however it is defined in those terms) must exist. Existence exists for "nothingness" to exist. Even if there were no consciousness , no perceptions, that condition (existence exists) would still have to apply for "nothingness" to exist.
I don't consider myself in any way existentialist and quite frankly don't see it has anything do with the bottom line philosophy here anyway. On the contrary, I am putting forward a rather non- existentialist position, which suggests stuff isn't entirely free, being dependant on existence.
My point is that by the philosophical argument you describe, "nothing" isn't in any different a category than all other concepts or things and cannot be any special link to knowledge you suggest.
On the other hand, existence is different. It is irreducible.
A step further nearer absolute.