The Big Bang Theory
According to this theory, the universe is expanding after sudden inflation of a very small point which had space, time, energy and matter squeezed together in a very small volume (known as Singularity).
Cosmic Microwave Background Radiation (CMBR) is the left over radiation from the Big Bang.
If a wave source moves towards us, the wavelength seems shorter to us; but the wavelength seems longer if the wave source is moving further away. This is known as the Doppler Effect, and all types of waves show this effect.
Scientists call such increase in wavelength a Red-Shift.
In contrast a decrease in wavelength is called Blue-Shift.
We observe an increase in wavelength or Red-Shift of light from most distant galaxies which means that they are moving away from us.
We have all experienced the siren of a fire engine - it appears high pitched as it approaches us and low pitched as it moves away from us (check this out http://www.youtube.com/watch?v=imoxDcn2Sgo).