The doppler effect
The changing of wavelength and frequency of waves relative to the observer

The source moves towards the observer, the observed wavelength decreases and and the frequency increases

galaxies are a large collection of stars

the faster the speed of the star, the greater the red-shift is

Bluue-shift- when a galaxy is moving towards us and the rays from the galaxy are moving towards the darker end of the spectrum

All the distant galaxies are moving away from each other, so the whole universe is expanding

The Big Bang Theory

The universe has been expanding ever since the big bang, high energy gamma rad. would have been produced
-As the universe expanded it would have become lower-energy rad.