Light From Other Galaxies is Red-Shifted
Chemical elements absorb different frequencies of light. This creates a type of pattern in the light spectrum. Light from galaxies has the same pattern, except the frequencies that are absorbed are shifted towards the red end of the spectrum. This is called red shift.
The further away the galaxies are, the more red shift they are. Scientists then assume that all of the galaxies must have all been together at some point, which then supports the Big Bang theory.
The Doppler Effect
The Doppler Effect is when the wavelengths and frequencies of waves appear to be different relative to the movement of the wave or the person listening.
So when a car drives past you, you hear its engine go higher in pitch. This isn't actually happening, but the apparent shortening of waves makes you hear a different pitch.
Cosmic Background Radiation is more evidence to back up the Big Bang theory.
Scientists have detected some microwave radiation that can not be accounted for. The theory is this:
When the Big Bang happened, light was released. As the universe stretched, these waves stretched with them turning the light from a very short wave lengths to another - which means they become microwaves.
CMBR comes from all parts of the universe, and the Big Bang is the only theory that can explain it.