And if you wonder, if money cannot be created or destroyed, how does a monetary system handle the population tripling over the past 50 years?
The answer is national debt. The US national debt is just an artifact of this double entry bookkeeping. In order for there to be +money in the economy for the ever-growing number of citizens to do commerce with, the treasury incurs on itself -money. That's national debt.
The corollary then, if the US ever pays back all of its national debt, the economy will crash because there just aren't cash available for people to do commerce with. A +90trillion on the government balance sheet equals a -90trillion on the private sector balance sheet.