He found a constant relationship between galaxies' redshift and distance, which helped to eventually prove that the universe is expanding
Redshift -
At least three types of redshift occur in the universe — from the universe's expansion, from the movement of galaxies relative to each other and from "gravitational redshift," which happens when light is shifted due to the massive amount of matter inside of a galaxy
happens when light or other electromagnetic radiation from an object is increased in the wavelength