N=1? You believe in Plate Tectonics, even though there is 1 Earth, but not the Big Bang because there is only 1 Universe? There are many reasons that lead to the conclusion of the Big Bang. The Doppler effect and Fraunhofer lines lead to the conclusion that everything in the Universe is flying away from our Galaxy. They had to start somewhere. If everything was once in one place, it would have been very hot, and you could predict that temperature. If the universe was that temperature at one time, the black body radiation produced should still be around. We searched for it, and found it exactly as predicted. Once you can predict something based on a hypothesis, and then confirm the prediction, you are on your way to a valid theory. Many more experiments have been conducted, refining and improving the theory.