Cosmologists want to know how everything began! They study the origin of the universe and how the universe has changed since then. Cosmology is a branch of astronomy and astrophysics. Today, cosmology has a large focus on the Big Bang theory, the dominant theory on the origin of the universe, which states that the universe began from a singularity, or a small point, and has expanded over the years (and is still expanding!) to form the universe we know today.  To pursue a career in cosmology, a doctorate in astronomy, astrophysics, or theoretical physics is usually required.