The cosmic microwave background (CMB) is the thermal remnant of the big bang. It was once a warm glow of about 3,000 K, but as the universe expanded and cooled the CMB gradually chilled to about 2.7 K. As a result, its once orange glow has shifted down to the microwave range, hence its name. Usually the CMB is presented as an image, where various colors represent the small fluctuations in the background, but when it was first observed it was detected as sound.
Because the CMB is in the microwave range, you can convert its electromagnetic waves to sound just as we do with radio. The result is a kind of faint static you can hear above. The sound was captured by Robert Wilson, who with Arno Penzias made the first measurement of the CMB as a blackbody background. It might not sound like much, but the very fact that it’s there was the first definitive proof of the big bang. Throughout the entire universe that faint sound can be heard. It’s everywhere and in all directions.
People often imagine the big bang as beginning with an explosion expanding out of darkness, but that faint hiss shows us that isn’t true. The big bang happened everywhere, and everything we see around us is a product of it.