The U.S. Dollar is and has been the world`s reserve currency for maybe 60, 70 years now. It took over from the Pound Sterling which had been the world`s reserve currency itself for 160 years.
Unfortunately the U.S. Dollar, and I am an American citizen, is a terribly flawed currency. We are the largest debtor in the world and the debts are going higher and higher. Countries that get themselves into these kind of situation ultimately have serious problems with their currencies and something comes along to replace it.