When you are between channels on an analog television, the snow that you see on the screen is made up of interference from background signals that the antenna on your TV is picking up. Some of the “snow” is from other transmissions here on Earth, and some is from other radio emissions from space. Part of that interference – about 1% or less – comes from background radiation leftover from the Big Bang, called the Cosmic Microwave Background. The same is true for FM radios – when the radio is tuned to a frequency that is between stations, part of the hiss that you hear, called “white noise”, is leftover radiation from the Big Bang some 13.7 billion years ago.
In 1965, Arno Penzias and Robert Wilson published a scientific study which proved that a radio signal with a wavelength of 7.3 cm was being emitted uniformly throughout all parts of the sky. This signal became known as cosmic microwave background radiation, CMB.
However, before all this, along with the particles of atoms (neutrons and electrons), particles of light (photons) where also formed. These photons, still hot from the Big Bang, could never fully interact with the atoms and are still cooling even now.
This residual heat is still present in the world today and because television can pick up stray electromagnetic waves, a TV set can show the residual radiation of the universe. TV static is anywhere from 1/3 to 1/4 background radiation.