Title: Why the US Dollar Is the World Currency
Headings: 
World Economy Trade Policy
Why the US Dollar Is the Global Currency
The U.S. Dollar Is the Strongest World Currency
Why the Dollar Is the Global Currency
Calls for a One World Currency
The Bottom Line
Article Sources

Content: In the foreign exchange market, the dollar rules. Around 90% of forex trading involves the U.S. dollar. The dollar is just one of the world's 185 currencies according to the International Standards Organization List, but most of these currencies are only used inside their own countries. 3
Theoretically, any one of them could replace the dollar as the world's currency, but they won't because they aren't as widely traded
Almost 40% of the world's debt is issued in dollars. 4 As a result, foreign banks need a lot of dollars to conduct business. This became evident during the 2008 financial crisis. Non-American banks had $27 trillion in international liabilities denominated in foreign currencies. Of that, $18 trillion was in U.S. dollars. 5 As a result, the U.S. Federal Reserve had to increase its dollar swap line. That was the only way to keep the world's banks from running out of dollars.