Although this is not how it arose historically, the most precise evidence for nonbaryonic dark matter now comes from the early universe. The is no place for "baryonic dark matter" to hide in the early universe: the temperature and density are high enough that all baryons are strongly coupled, and so all baryonic matter is the same.

Early-universe measurements are consistent with baryons comprising about 5% of the energy density within the present-day Universe and much more weakly coupled nonbaryonic matter comprising about 25%. If the abundance of baryonic matter were much higher than that, then:

1. The relative abundances of light elements/isotopes that emerged from primordial nucleosynthesis would be very different. For example, the fractional abundance of deuterium would be a lot lower, because with more baryons, there is more time for the deuterium to fuse into helium before the density of baryons drops too low.
2. Sound waves in the primordial plasma would have much higher amplitudes, since sound can transmit through baryons but not through nonbaryonic dark matter. That would leave a clear imprint in the pattern of temperature fluctuations in the cosmic microwave background.

Although this is not how it arose historically, the most precise evidence for nonbaryonic dark matter now comes from the early universe. The is no place for "baryonic dark matter" to hide in the early universe: the temperature and density are high enough that all baryons are strongly coupled, and so all baryonic matter is the same.

Inferences based on early-universe physics are consistent with baryons comprising about 5% of the energy density within the present-day universe and nonbaryonic dark matter comprising about 26%. If the abundance of baryonic matter were much higher than that, then:

1. The relative abundances of light elements/isotopes that emerged from primordial nucleosynthesis would be very different. For example, the fractional abundance of deuterium would be a lot lower, because with more baryons, there is more time for the deuterium to fuse into helium before the density of baryons drops too low.
2. Sound waves in the primordial plasma would have much higher amplitudes, since sound can transmit through baryons but not through nonbaryonic dark matter. That would leave a clear imprint in the pattern of temperature fluctuations in the cosmic microwave background.