Re: Why does the universe need dark matter?

You already sound very well informed on this issue!

In fact, in standard Big-Bang nucleosynthesis, the ratio of protons to
neutrons is determined partly by the small mass difference between the
two particles (which slightly favours the production of lighter protons
over heavier neutrons), and then, as you say, by the decay of neutrons
during the first few minutes while most were combining to form first
Deuterium and then Helium.

The strongest observational constraints at the present time come from
observation of Deuterium rather than Helium (although the 25% by mass
He fraction is itself an important confirmation of the overall picture).
Deuterium, an isotope of Hydrogen with one proton and one neutron, is 
much rarer, but has been measured by the absorption lines it produces
in the spectra of distant quasars.

The conclusion is that the density of baryons in the present universe 
must be about 4x10e-28 (4 times 10 to the power -28) kg per cubic metre.
This about a fifth of a proton per cubic metre on average...not very 
dense! (It is a few percent of the density required to close the universe).

This density is only about 10 or 20% of the density generally inferred
from measurements based on the gravitational attractions of galaxies
and clusters of galaxies. This is why there seems to be a requirement
for some form of dark matter which is not the usual run-of-the-mill
matter we are familiar with.