Astronomers explain the phenomenon of an expanding universe primarily through the observation of redshift in distant galaxies, which indicates that these galaxies are moving away from us. This redshift occurs because the light emitted from these galaxies stretches to longer wavelengths as they recede, similar to the Doppler effect experienced with sound waves. This discovery was first noted by Edwin Hubble in the 1920s, leading to the formulation of Hubble’s Law, which states that the velocity at which a galaxy is moving away is proportional to its distance from us.
This expansion supports 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. As the universe expands, it also cools, allowing for the formation of galaxies and other cosmic structures over billions of years. Additionally, recent observations of cosmic microwave background radiation further confirm this model by revealing remnants of the early universe, thereby solidifying our understanding of an ever-expanding cosmos.