They are backed by real dollars that they have in cash reserve in a bank. At least in theory. What is currently happening is because of too much volume they ha

They are backed by real dollars that they have in cash reserve in a bank. At least in theory. What is currently happening is because of too much volume they have hard time putting more money back into the ecosystem (which is the good scenario, it means they have money but things are going just a bit slow), or they are running out of money (which is a bad scenario that would cause many things to go down with it).