The blockchain is not determined by a majority vote and does not necessarily change if a significant portion of nodes claim an alternative chain is the main chain. Rather the main chain is determined by each node independently, and all nodes arrive at the same conclusion independently because they all are following the same set of rules.
So even if an attacker were able to have more nodes than honest nodes, and those nodes were broadcasting an alternate chain, all of the blocks in that chain would still have to pass validation by the honest nodes before those honest nodes will accept those blocks. This means that all transactions must be valid, the proof of work for every block must be valid, etc. That alternative chain must be valid for it to be accepted by other nodes, regardless of how many malicious nodes claim that it is the main chain.
If the alternative chain contained "fake data" that is invalid, then the entire chain would be rejected as invalid and no honest node would accept it. If the alternative chain is fully valid, and if it has more cumulative work than what honest nodes currently consider to be the main chain, then the honest nodes would switch to that alternative chain.