The Net Advance of Physics: The Nature of Dark Matter, by Kim Griest -- Section 2D.

The Baryonic Content of the Universe

An important ingredient in the motivation for non-baryonic dark

matter comes from big bang nucleosynthesis limits on the average

baryonic content of the Universe. To agree with the measured

abundances of helium, deuterium, and lithium, the baryonic

content of the Universe must be between [13,

14, 15]. Given the large uncertainty in h this means .

These values are far below unity, so the theoretical predilection for

(or the observational evidence for ) forces the bulk

of the dark matter to be non-baryonic. The lower limit of this

range is actually above the abundance of known stars, gas, etc., and

so there also seems to be evidence for substantial baryonic dark

matter as well.

However, if one considered only the most secure dark matter, that

found in spiral galaxies, then it is completely possible that it is all

baryonic. Since this is the only dark matter which is directly

accessible to experimental detection, it is crucial to consider the

possibility of an entirely baryonic dark halo.

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