The Fading Fireball of the Big Bang refers to a cosmic microwave background that we can observe anywhere in space. Right after the Big Bang, the universe was flooded with incredible bright light. As the universe expanded over billions of years, the expansion of space shifted the wavelength of this remaining bright light into red light, later infrared light, and eventually into microwaves. There is no direct proof that this light remains from the Big Bang, however, there are other indications that the Big Bang happened.

If we look into space, we are actually looking back in time. Objects we observe are so far away that their light needs millions or billions of years to reach us. This means that the light we are seeing today shows stars in their state millions of years ago. Modern telescopes are so powerful that they can observe light from billions of light years away, close to the time of the Big Bang happened. The Big Bang predicts gas clouds of hydrogen and helium that existed before the formation of stars. In fact, astronomers observe such gas clouds about 12 to 13 billion light years away from us and use spectroscopy to determine that these clouds consist of hydrogen and helium. Being able to look back in time this way suggests that the Big Bang happened and that the cosmic microwave background surrounding us is in fact a result of the Big Bang.