From the 1970s to today, most countries have run on a system of fiat money, which is government-issued money that is not backed by a commodity. The US dollar is fiat money, which means it is backed by the government, but not by any physical asset. The value of money is set by supply and demand for paper money, as well as supply and demand for other goods and services in the economy. The prices for those goods and services, including gold and silver, can fluctuate based on market conditions.