What backs the U.S. dollar today?

I'm curious to know, what exactly backs the U.S. dollar in today's economic landscape? Is it still tied to the gold standard as it was in the past, or has it evolved to rely on something else? Given the complex nature of global finance and the role of the Federal Reserve, how does the value and stability of the U.S. dollar remain intact in the face of various economic factors and challenges? I'm eager to understand the underpinnings that support the world's most widely used currency.