Edwin Hubble observed that the light from distant galaxies was redshifted, indicating they were moving away from us. The further away the galaxy, the greater the redshift, meaning more distant galaxies were receding faster. This led to the conclusion that the universe is expanding, with all galaxies moving away from each other. While the expansion provides evidence for the big bang theory, it also raises questions about dark matter providing more gravity than visible matter, as well as the acceleration of the expansion due to dark energy.