Expanding Universe16.4 - Understand the evidence to confirm the discovery of the expanding universe
This section largely covers the idea that the universe is expanding.
Edwin Hubble proved that galaxies are moving away from each other because they show a redshift (see the Doppler page). Evidence of redshift galaxies shows us that it is still expanding.
Astonomers know that the Milky Way is not the centre of the Universe and so these galaxies are expanding in distance between each other.
This knowledge led scientists to theorise about the age of the Universe, how much smaller it was than it is now, and how was it formed.
Take a balloon that hasn't been blown up and draw some small dots on it. Now blow it up. You will see that the dots move further away from each other. Imagine these dots are galaxies and you can see how they are moving away from each other.