On Mon, 1 Oct 2001, michael wrote:
> hi im a student at lancaster university england. i'm currently writing a 
> report on dark matter. Is there anyway to work out what percentage of dark 
> matter is baryonic and what percent non-baryonic and if so how is it done.
> thanks.
> Mike.


Hi, Michael.  That is a very good question and one that is
at the heart of the matter.  Scientists do indeed believe
that the contributions from both non-baryonic and baryonic
dark matter are able to be stated.

In general, it is thought that it is almost impossible for a
significant portion of the dark matter to be baryonic.  At best,
a few percent of the missing mass can be ordinary baryonic matter.

The best piece of evidence for this is Big Bang Nucleosynthesis
(BBN) calculations. Given the knowledge of nuclear reaction rates,
the production of light elements in the first moments of the universe
can be calculated. These calculated abundances can be compared to
measurements of the abundances in the universe.  Theory matches
observation very well -- if only a few percent of the mass of the
universe is baryonic. If there were much more baryonic mass, the
calculations would be thrown off.  

Do note, however, that even of the few percent of baryonic mass,
much of it is missing too.  This is believed to be in the form of
dust or ionized gas or MACHO-like objects.  So, there are in fact
two missing mass problems -- the missing baryonic mass and the real
missing mass problem, which must be solved by a non-baryonic particle.

Just for the record, about 2/3 of the mass-energy of the universe
is dark energy.  1/3 is dark matter and a marginal percentage is
baryonic.

Yours,
Michael Scott Armel
Center for Particle Astrophysics
UC Berkeley