" Without major changes to the Big Bang theory itself, BBN will result in mass abundances of about 75% of hydrogen-1, about 25%
helium-4, about 0.01% of deuterium and
helium-3, trace amounts (on the order of 10−10) of lithium, and negligible heavier elements. That the observed abundances in the universe are generally consistent with these abundance numbers is considered strong evidence for the Big Bang theory."
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In other words, the mathematical theory, the big bang model, predicts ratios of elements, that we observe. With telescopes and spectrographic analysis.
Observation confirms theoretical prediction.