Big Bang Theory

The Big Bang Theory is the leading explanation for how the universe began. Simply put, it says the universe as we know it started with an infinitely hot and dense single point that inflated and stretched — first at unimaginable speeds, and then at a more measurable rate — over the next 13.8 billion years to the still-expanding cosmos that we know today.

The cosmic microwave background (CMB) is leftover radiation from the Big Bang or the time when the universe began. As the theory goes, when the universe was born it underwent rapid inflation, expansion and cooling. (The universe is still expanding today, and the expansion rate appears different depending on where you look). The CMB represents the heat leftover from the Big Bang.

You can't see the CMB with your naked eye, but it is everywhere in the universe. It is invisible to humans because it is so cold, just 2.725 degrees above absolute zero (minus 459.67 degrees Fahrenheit, or minus 273.15 degrees Celsius.) This means its radiation is most visible in the microwave part of the electromagnetic spectrum.