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.
According to NASA, CMB fills the universe and in the days before cable TV every household with television could see the afterglow of the Big Bang. By turning the television to an “in-between” channel, you could see the CMB as a static signal on the screen.