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Cosmologists know that the universe is expanding now, and extrapolate this expansion backwards in time in order to study what the early universe was like. About 13.75 billion years ago, all of the contents and energy in the universe was contained in a singularity with infinite density and temperature. It began to expand rapidly and this expansion is known as the Big Bang. The laws of physics as we know them did not apply during the early seconds of the universe and scientists can only speculate about what the early universe was like.