What is the evidence for the Big Bang theory?

Absorption spectra from distant galaxies consist of bands of coloured light crossed with black lines. The wavelengths of these black lines are shifted towards the red end of the spectrum when compared with light from similar sources in the laboratory. The black lines from more distant galaxies are therefore more "red-shifted", and this is due to the expansion of space itself. This evidence suggests that the Universe began its existence at a single point and has expanded outwards ever since.
Cosmic microwave background radiation, or "CMBR", on the other hand initially existed as very small wavelength radiation, such as gamma radiation, but an expanding Universe has caused the wavelength to increase into the microwave region of the electromagnetic spectrum.