The fish bowl argument is the observation that things are the way they are not due to some improbable (or otherwise) happenstance but rather because we are completely unfamiliar with the alternative. The classic example is two fish in a fish bowl who will never know that there is any sort of a world whatsoever outside of their small glass bowl because they've never seen it. As far as they are concerned, the fish bowl is their reality. If fish could talk, and you asked those fish to describe what the word "dry" means, assuming the fish haven't had any traumatic experiences, they wouldn't begin to understand because they've spent their whole lives wet.
Applying that to the universe, we say "Why do we exist?" and of course the trivial fish bowl argument response to that question is "If we didn't exist, we wouldn't be here to question if we exist."
This leaves me to question if we really know what nonexistence means. For example, can we truly imagine a color which has never existed? Are we really capable of determining if something exists or not, since we've never actually placed eyes on something which doesn't exist? You could argue that you can imagine an elephant with wings and since such animals don't exist that one can determine what exists and what doesn't, though it's not true that it doesn't exist necessarily. On some planet in a galaxy far away, perhaps there exist elephant-like creatures with wings. I could go so far as to argue that if the universe is infinite, it isn't possible that something does not exist (assuming of course the very definition itself isn't contradictory like a triangle with four sides).
Therefore I can only conclude that we can only determine what exists. We cannot determine what does not exist on this basis. We know that the set of things which do not exist is everything which is not contained in the set of things which do exist, by definition of existence, though we cannot determine its characteristics. We cannot even determine if the set of things which do not exist is empty or not.
I know this might seem like an odd question, but let me just say that I'm not chomping on cheetos and smoking mary jane. Is this logic sound or perhaps trivially so, or am I missing something?