who was the first scientist to prove the existence of galaxies beyond the milky way?

Answer 1

How did we learn that when we look out into space we look back into time? This discovery was primarily the work of American astronomer Edwin Hubble (1889-1953). When he trained the new 100-inch telescope at the Mt. Wilson Observatory on the heavens in the mid-l920s, it was commonly assumed that our Milky Way Galaxy was pretty much all that existed in the universe. Then, he and other astronomers produced the astonishing evidence that ours is but one of countless galaxies, "islands" widely separated by immense stretches of space in a universe enormously larger than they had assumed. Furthermore, in whatever direction in space Hubble looked, he discovered that aside from our nearest neighbor Andromeda, all of the other galaxies were moving away from ours. He was able to determine this fact by measuring the shift in the signature spectra of certain ionized atoms in the stars of distant galaxies as they receded from our own galaxy: the more proportionately broad the shift, the further the distance. Hubble's law, as it came to be called, expresses a direct and proportionate relationship between the distance and the velocity of another galaxy relative to our own: the further away the galaxy, the faster it appears to be moving (Silk, 1994, 32-35; Morris 13-19).

Answer 2

Hubble.