。
Nobody knows what will happen tomorrow.
Literal
Tomorrow what [subject-が] occur [question-か] nobody [to-に] know-not.
誰にもわからない — 'nobody knows' — uses にも with the negative to create universal negation. Compared with the parallel rhetorical question form 誰にわかるのか ('who could possibly know?'), this is the direct statement of the same idea, asserting what the rhetorical question implies.