The timeline of our observable Universe's history, where the observable portion expands to larger and larger sizes as we move forward in time away from the Big Bang.NASA / WMAP SCIENCE TEAM

1940s — While the vast majority of experimental and observational resources went into spy satellites, rocketry and the development of nuclear technology, theoretical physicists were still hard at work. In 1945, George Gamow made the ultimate extrapolation of the expanding Universe: if the Universe is expanding and cooling today, then it must have been hotter and denser in the past. Going backwards, there must have been a time where it was so hot and dense that neutral atoms couldn’t form, and before that where atomic nuclei couldn’t form. If this were true, then before any stars ever formed, that material the Universe began with should have a specific ratio of the lightest elements, and there ought to be a leftover glow permeating all directions in the Universe just a few degrees above absolute zero today. This framework is today known as the Big Bang, and was the greatest idea to come out of the 1940s.