Cosmic expansion refers to the phenomenon where the universe is continuously growing larger over time. This process was first observed by astronomer Edwin Hubble in the 1920s, who discovered that distant galaxies are moving away from us. The farther a galaxy is, the faster it appears to be receding, indicating that space itself is expanding. This expansion is a key aspect of the Big Bang theory, which suggests that the universe began as a hot, dense point and has been expanding ever since. The rate of this expansion is measured by the Hubble constant, which helps scientists understand the universe's age and its ultimate fate.