If I am to assume lack of proof of existence leads to limbo (uncertainty whether something does or does not exist) then it very quickly becomes a philosophical topic. The only certainty is existence and proof of the latter.
It doesn't have to be material, even - how do we know emotions exist? Hate, anger, happiness, love? Billions of people have no reason to lie to me or anyone else and put on an act - the only logical conclusion is that these emotions do exist, we can all feel them.
What I've been saying is: "lack of proof means non-existence" in math exclusively - we are welcome to philosophize about what you or I or they hold true or theologize about the existence of god all we want, until there is solid proof, it stays out of the math field.
You don't go to work because you MAY be paid for your work - the only certainty you accept is you WILL get paid and you have proof of it from many other co-workers who also get paid - the money does exist regardless, however your salary will only exist once you get paid.
Should you not get paid, therefore you can't prove your salary exists - so you can't convince the bank to give you a house loan or all purpose loan either. If there is no proof, it does not exist.
Or maybe you like to deal a lot with maybes and I am the crazy one, works for me either way.
Might also be necessary - I don't think of math as punching numbers and theories together - to me, math is logic. So to answer the original question of the topic (not worded as a yes-or-no question, but essentially comes down to it) - No - I don't care for "none-that-we-know-of"s - Either 1 or 0. If you can't prove it, it's a 0, if you prove it's a 0, it's a 0 and naturally, if you prove it's a 1, it's a 1.