There are many fantastic and well referenced free documentaries and short films online about money creation of which everybody should watch at least two or three in order to understand how money comes into being. Money is at the centre of almost every national culture whether we like it or not and it's quite remarkable how many people don't know where it actually comes from.
The short explanation is that money is loaned into existence by banks as debt. Banks keep very little money themselves and instead keep small reserves because they know that not everybody who deposits money with them for safe keeping will want their money back at the same time. Only about 3% of money is actual physical currency. the rest is contained in digital form on computers.
This system is called 'Fractional Reserve Banking' and is the model which has been used globally since 1971 when the U.S. stopped linking the value of the dollar to gold. Before this time the money supply was restricted by the amount of gold owned by the issuers of money. Later in the mid-1990s the U.S. then repealed the 'Glass-Steagle Act' which had prevented banks from using depositors funds to speculate on global markets. These two events together mean that international financial institutions now have a potentially unlimited supply of money to 'play' with on the global markets.