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Big Bang. Conceptual computer artwork of the origin and evolution of the universe. The term Big Bang describes the initial expansion of all matter in the universe from an infinitely compact state 13.7 billion years ago. The initial conditions are not known, but less than a second after the beginning, temperatures were trillions of degrees Celsius and the primordial universe was much smaller than an atom. It has been expanding and cooling ever since. (KEYSTONE/SCIENCE PHOTO LIBRARY/JEAN-FRANCOIS PODEVIN/SCIENCE PHOTO LIBRARY)