A Catholic priest proposed the expanding universe before observations confirmed it

In 1927, Georges Lemaître — a Belgian physicist and ordained priest — derived from Einstein's equations that the universe must be expanding, two years before Edwin Hubble's data backed it up. In 1931 he went further, proposing all matter once began as a single 'primeval atom' — the first real statement of what we now call the Big Bang.

— Georges Lemaître, A Homogeneous Universe of Constant Mass and Increasing Radius Accounting for the Radial Velocity of Extra-Galactic Nebulae — Annales de la Société Scientifique de Bruxelles, 1927; 'primeval atom' hypothesis, 1931