Believe it or not, a small pecentage of it is the reverberations from the big bang .
For more about this read on(from the web):
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. Penzias and Wilson won the Nobel Prize in physics in 1978 for their findings. It is believed that this cosmic static is comprised of photons of energy that are still cooling 15 billion years after the Big Bang. If you turn a television to a channel with no station, you will see this static and given the level of intelligence of most TV programs, this static is about the only thing worth watching.
The Big Bang is the theory about the creation of the universe. This idea holds that some cataclysmic event caused all energy and space to be formed at once. Energy and space was fine, but no matter could exist because after such an explosion, the universe was still too hot. Over the course of time, the universe began to cool enough so that neutrons and electrons could attach to each other and form atoms which then became matter. Matter eventually would become planets and then atmospheres and then plants and then dinosaurs and mammals and then people and then Baywatch.
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.
How is it possible that a television set can actually show cooling photons? Well, lets look a little closer at how the modern television works.
In 1925, Philo Farnsworth perfected a system he called "image dissection" which formed the backbone of modern television. Farnsworth's original dissector would break down an image into 150 lines and then scan it 30 times per second. The transmittable image he produced was much cleaner and clearer than anything researchers had done until that point. Ironically, the image Farnsworth chose to send of his new invention was the dollar sign symbol.
This image scanning technique, called interlacing, is how the TV shows movement. Small red, green and blue dots of light, called pixels, are painted on the TV screen 30 times per second. This tri-beam of color is produced by an "electron gun" which actually shoots colors onto the screen and then into your eyeball. Most modern TVs usually have around 525 lines, which means that since each line is painted in color 30 times a second, then the electron gun paints 15,750 lines per second. This is why images seem to move smoothly around the screen.
When the television shows static it is showing the ever-present background radation, so the electron gun is shooting colors at the screen in a random order.
When you are watching TV, you are watching static. The TV signal is not perfect, there are always small random electrons that go astray when the images are shot onto the screen. Digital television has improved on the problem, but you will always be exposed to the random fuzz in a normally fluid signal. You may not see it, but the background radiation is always there. So the static is always there.
Watching static on TV will give you a headache, so watching TV programs will give you a headache. Television is often referred to as "mindless entertainment." This has got to be true, because if you try and engage your brain, it's liable to start hurting