Galaxy expansion refers to the phenomenon where galaxies are moving away from each other over time. This observation is primarily based on the redshift of light from distant galaxies, which indicates that they are receding. The farther away a galaxy is, the faster it appears to be moving away, a relationship known as Hubble's Law, named after astronomer Edwin Hubble. The expansion of the universe began with the Big Bang, approximately 13.8 billion years ago. As the universe continues to expand, it affects the distribution of galaxies and cosmic structures. This expansion is driven by a mysterious force called dark energy, which is believed to make up about 68% of the universe.