This is evidenced by Doppler
shift effect. This is the phenomenon where waves that are coming from an object
that is accelerating towards your direction are heard at a higher frequency than that of
objects moving away from you. This is the same for light (which also has
wave-like properties). When observing distant galaxies, astronomers discovered that
they all showed to be redshift (change in the frequency of light towards red
color). This means that all galaxies are moving away from each other because
the universe is expanding.
