Light from a star or galaxy contains a pattern of dark lines at particular wavelengths. When a galaxy is moving away from us, every wavelength in its light is stretched, so the pattern appears at longer wavelengths than it does for a source at rest. A longer wavelength means a lower frequency. Visible light with a longer wavelength is nearer the red end of the spectrum, so the light is said to be red-shifted. The effect is similar to the pitch of an ambulance siren dropping as it moves away from you.
Astronomers find that light from nearly every distant galaxy is red-shifted, so nearly all galaxies are receding from us. They also find that the further away a galaxy is, the greater its red-shift, which means the faster it is moving away. This change of speed with distance is evidence that space is expanding. Because the whole universe is expanding, an observer in any other galaxy would also see the rest moving away, so we are not at a special centre.
