Unknowable is that which can never be Experience|experienced and so will never get organised in the Memory.
Unknowable is a state of Knowledge.
Unknowable is a kind of Knowledge.
There is a difference between there being something and knowability of it. We may be aware of something, but we may not be able to experience it and make it Known. Unknowable is different from Unknown as unknown has a potential to become known but the unknowable will never be known.