I think there is no money in the bank because the money is being used or invested by the bank without our knowledge, because the regulations are clear that banks can use their customers' money. I don't know what percentage the amount is but it can be done by them, maybe the regulations in each country are slightly different.
I don't believe that's how they do it with the money of their clients, banks work on a fractional reserve banking system which means that they only keep a part of the deposited money in the bank while they chip in a lot of it so they can lend it to borrowers, that's why banks can close prematurely during their operating hours especially at a time where a lot of people have withdrawn their money from the bank, they won't be able to accommodate everyone. Another reason why they work is because as I'd hate to say it, banks work on trust ever since, trust that they'll not invalidate you've got money in them by proving it through a piece of paper.