The "Expanding Universe" refers to the observation that galaxies are moving away from each other, indicating that the universe is growing larger over time. This phenomenon was first noted by astronomer Edwin Hubble in the 1920s, who discovered that distant galaxies exhibit a redshift, meaning their light is stretched as they move away. This redshift is a key piece of evidence supporting the theory of an expanding universe. The expansion of the universe is often explained by the Big Bang theory, which posits that the universe began as a singularity approximately 13.8 billion years ago. As the universe expanded, it cooled, allowing matter to form into stars and galaxies. Today, scientists continue to study the rate of this expansion, which is influenced by factors such as dark energy and gravity.