In article , "Richard D. Saam"
writes:

OK, due to constraints from
primordial nucleosynthesis, we now know that most of the dark matter is
non-baryonic
I would like to hear the logic underlying this statement.

One parameter is the photon-to-baryon ratio. We can count the photons,
so we know the total number of baryons. (Almost all photons are in the
CMB, by the way.)

The photon density rho_p = 413/cm^3
reflects the Black Body calculation at 2.73 K
and actual photon counting (WMAP)
The critical density rho_c is separately calculated
at (3/8pi)*H^2/G = 9.56E-30 g/cc

It is understood that
rho_c*(1+z)^3/rho_p*(1+z)^3
is constant
and expressed in terms of hydrogen atoms
rho_c*(1+z)^3/(Avogadro*rho_p*(1+z)^3)
is constant

We can elimate (1+z)^3 from the above and just say that the ratio is
constant.

and that .0464 (WMAP) of critical density is baryonic
as measured by Baryonic Acoustic Oscillation at z=1100.

We know the critical density, we know the fraction of baryons and thus
we know the number of baryons.

It would appear that knowing the photon density rho_p
does not dictate the number of baryons.

What I meant was that the predictions of big-bang nucleosynthesis depend
on this ratio, thus knowing the ratio ties down the predictions so that
there is no leeway.