At the start of the 20th century, astronomer Edwin Hubble observed that light coming from distant galaxies appeared redder than it should be.
But how can you tell what it should be like? How do you know that it appears redder?
We have all heard ambulances go past us, sirens blaring. As they move away, the pitch of the siren gets lower. The same is true for car engines:
Coming towards you, the sound waves are squashed closer together (shorter wavelength), so have a higher pitch. Moving away from you, the waves are pulled apart (longer), so are lower.
The same is true for light. Coming towards you, they appear bluer. Going away, redder. This is called Doppler Shift.