In this episode of Everything Explained, we unpack how scientists determined the age of the Universe—and why that famous number, 13.8 billion years, is the result of multiple lines of evidence working together. From galaxies racing away from each other to the faint afterglow of the Big Bang known as the cosmic microwave background, we explore the key clues that let cosmologists rewind cosmic history. The episode covers expansion rates, standard candles like supernovae, primordial elements forged in the first minutes, and the subtle ripples imprinted across the cosmos. Rather than a single measurement, the age of the Universe emerges as a conversation between independent methods that all converge on the same deep past—revealing how patiently gathered evidence can illuminate the very beginning of everything.