What observational evidence supports the fact that our universe is expanding?
In 1920 Edwin Hubble photographed different galaxies and studied their spectrum.
He found the red shifts of galaxies and for more distant galaxies showing higher red shifts..This is the proof of expansion of universe.
Later type Ia supernovas were studied and they show that the universe is not only expanding but accelerating.Cosmologists assume that it is due to dark energy..
picture credit astroiku edu.