The "Expansion of the Universe" refers to the observation that galaxies are moving away from each other over time. This phenomenon was first noted by astronomer Edwin Hubble in the 1920s, who discovered that distant galaxies exhibit a redshift, indicating they are receding. This suggests that the universe is not static but is continuously growing larger. The expansion is often explained by the Big Bang theory, which posits that the universe began as a singularity approximately 13.8 billion years ago and has been expanding ever since. This expansion is not uniform; some regions of space expand faster than others, influenced by factors like dark energy.