Firstly, Hubble figured out that the Milky Way is not the only galaxy.

Secondly, Hubble discovered that the galaxies are constantly moving away from each other in all the directions.his expansion is evidenced by the redshift of light from distant galaxies, indicating that the farther a galaxy is from us, the faster it is receding. 

Hubble's findings provided strong support for the Big Bang theory, suggesting that the universe had a beginning and has been growing ever since. This discovery fundamentally changed our understanding of the cosmos and laid the groundwork for modern cosmology.