The place or space which lies beyond all known possibility and comprehension of man or god.
This is believed to be the place from where only the Divine can pass into during his time of absence from the Limited World. However, given there's no ability for anyone to possibly understand the laws or reality of such a plain of existence, such ideas amongst man are merely speculation as no one can even say with any certainty if it actually exists.
There is nothing more to say about the Unknown, other than it's merely just that: The Unknown. It may exist and, if it does, we will never know what it is or anything else about it.