What is the main evidence for the Big Bang theory?

The main evidence that we currently have to support the Big Bang Theory is the cosmic background radiation. This is electromagnetic radiation that appears to be evenly spread throughout the universe. It is mostly in the microwave spectrum, equating to a temperature of about 3K. This suggests that the universe was once very hot and compressed, and as the universe has expanded, the once very high energy, short wavelength radiation has red-shifted to lower energy, longer wavelength microwave radiation.