If science is based on observable, measurable data, what is the basis of science

If science is based on observable, measurable data, what is the basis of science

If science is based on observable, measurable data, what is the basis of science's belief of the origins of the 'Big Bang'? Even religions talk of the cataclysmic beginnings of the Universe, but they don't claim the Bang was of Nothing. Observable, mathematical data suggests nothing begets nothing.

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