Edwin Hubble used a telescope and observed fuzzy blobs of light off in the distance. He realized that these lights were:

A. Stars

B. Galaxies

C. Universes

D. Planets

He realized they were galaxies. By this observation, he was actually able to realize that there is a universe of galaxies outside our Milky Way and he was able to proof that the universe is expanding, by measuring the redshift of the light coming from these galaxies: the larger the redshift, the larger the distance of the galaxy from us. This result is also referred to as "Hubble's law".