
In the 1920s, Belgian priest and physicist Georges Lemaître first proposed that the Universe was expanding from an initial point. In 1929, Edwin Hubble confirmed this by discovering the recession of galaxies.
Imagine baking raisin bread. The dough rises, and the raisins move apart. The farther apart two raisins are, the faster they recede. So it is with galaxies: the expansion of space itself pushes them apart, and we observe the redshift of their light.