A noun for something that you cannot prove; something you can never be sure of actually happening/being real?
closed as off-topic by Tushar Raj, michael.hor257k, Lawrence, tmgr, AmE speaker Jan 9 at 3:47
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One of the many synonymous is uncertainty.
I tend to think incertitude is up to the task without the bulky feeling.
in·cer·ti·tude | (ˌ)in-ˈsər-tə-ˌtüd, -ˌtyüd\
Definition of incertitude
a : absence of assurance or confidence : DOUBT
b : the quality or state of being unstable or insecure
Consider inscrutability, which is the noun form of inscrutable:
Impossible to understand or interpret.
Oxford Living Dictionaries
It's mostly used to describe people that are difficult to understand, but it can be applied generally as well. Consider this second definition source:
the quality of being impossible to investigate
“the inscrutability of the future”