  It is proposed that when the era of the big-bang nucleosynthesis ended,
almost all of the 75 percent of the observed total baryonic matter remained in
the form of hydrogen and continued to exist in the form of protons and
electrons. They are present today as baryonic dark matter in the form of
intergalactic hydrogen plasma. To test our hypothesis we have investigated the
effects of Thomson scattering by free electrons on the reported dimming of Type
Ia supernovae. The quantitative results of our calculation suggest that the
dimming of these supernovae, which are dimmer than expected and hence more
distant than predicted by Hubble expansion, is a result of Thomson scattering
without cosmic acceleration.
