Final answer:
Georges Lemaitre is credited with proposing the Big Bang Theory, which describes the beginning of the universe as a single dense entity that exploded and fragmented into atoms.
Step-by-step explanation:
The Big Bang Theory was first proposed by the Belgian priest and cosmologist Georges Lemaitre. He envisioned the universe starting in a single dense entity called the primeval atom, which then exploded and fragmented into the atoms we see today. Lemaitre's theory was further developed by scientists Alpher and Gamow, who worked on the physics of the Big Bang.