The expansion of the universe refers to the phenomenon where galaxies are moving away from each other, indicating that the universe is growing larger over time. This discovery, first observed by Edwin Hubble in the 1920s, suggests that the universe began from a hot, dense state known as the Big Bang and has been expanding ever since. As the universe expands, the space between galaxies increases, leading to a redshift in the light they emit. This redshift is a key piece of evidence supporting the theory of expansion, as it shows that distant galaxies are receding from us, providing insights into the universe's age and its ultimate fate.