Incapable of being known; not capable of being ascertained or discovered; above or beyond knowledge.
Unknown.
WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
adj. not knowable
Word Usage
"None of us could have known what it would be like (yes, Bub, unknowable is exactly the right word) – distracted by shiny things or not – and most of us understood that from the beginning."