Hello, new to the group. Just a point of order, really. The universe, by definition, is entirety. So, does it really make any sense to talk about "other universes" or "multiverses".

Could we refer to the unknown as any part of the (single) unobservable universe?

As to the general idea of omniscience, normally associated with "God", I believe it to be theoretically possible, provided it does not involve a logical contradiction, eg "Can you know what the highest prime number is?" [because that involves an infinity, which by its very nature, cannot be actualised.].