I am surprised that no one has mentioned Big Bang nucleosynthesis.
The theory of Big Bang nucleosynthesis together with measurements of, e.g., the ratio of the primordial density of deuterium to the primordial density of hydrogen put the baryonic (protons and neutrons and their exotic cousins) content of the universe at about 15% of the total matter content of the universe given by standard interpretation of
results like those from WMAP.
According to the standard cosmological model, the majority of dark matter is not normal matter.