We reviewed and discussed primordial nucleosynthesis, element abundances of the elements made in the first 3 minutes of the Universe and what we can learn from them (mainly He4 and D), the density of baryonic matter in the Universe, measuring the density of all matter (mean velocity [or velocity dispersion] of galaxies in clusters, rotation velocity of individual spiral galaxies) in the Universe, why we think we know that non-baryonic dark matter must exist, and the era of recombination and the microwave background.
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