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Hubble's Law (OCR 21C)*.
The theory Hubble proposed in 1929 is called Hubble's Law, it says that:
The speed at which galaxies are moving away from us is proportional to their
distance from us (i.e. the faster a galaxy is moving, the further away it is)
He developed this law by observing and discovering that light from galaxies
that were further away from us had even longer wavelengths.
If galaxies are moving away from us space must be expanding.
*this is higher tier information.