🚀 go-pugleaf

Dale Bass and others object to the hypothesizing of loads of extra
dark matter lying about in order to fix up our various estimates
of the matter density, etc.  I have sympathy for this, especially
when it comes to postulating Mystery Particles (tm) but one thing
is that there is good reason to believe in a fair amount of
baryonic dark matter.

Fairly respectable nucleosynthesis calculations, which of course 
grow out of a hot big bang, but otherwise involve generally
accepted nuclear physics, imply that Omega_baryonic is in the
range 0.05 - 0.2.  (The factor of 4 is due solely to a Hubble
constant squared; that's uncertain to a factor of 2.)  This is,
depending on who you ask, 10 or more times the amount of baryons
one would infer from the luminosity density of the universe.  Hence,
baryonic dark matter.

Somebody just quoted Jerry Ostriker to me: "In astronomy, the only
real mass determination is a dynamical determination."  Just 'cause
we can't see it doesn't mean it's not there.

Robert Firth, your idea of a modified gravity law that has a different
dependence at large R has been tested, to some extent.  Surprisingly
one can probably rule it out.  An alternative which does kick around is
Mordechai Milgrom's "Modified Newtonian Dynamics," which rolls over
at small accelerations as opposed to large R.  Look his papers up for
details.