In article , Jos bergervoet
writes:

"Just how big is the Andromeda galaxy?" (astronomy.com)

http://astronomy.com/news/magazine/2018/02/adromeda-is-the-same-size-as-the-milky-way

If the mass of this very nearby galaxy is already difficult to
measure, does this mean that the total baryonic mass in the
universe is in fact also known only with, say, a factor 2 of
experimental error?

[And could this total error be in the other direction, thereby
reducing the amount of required dark matter?]

The best constraints on the baryon density come from big-bang
nucleosynthesis and the CMB. These are theoretically sound and based
only on standard physics. These indicate that there is MORE baryonic
matter than we know about, by a factor of a few. There are some
indications that it might be in cold gas between galaxies. Even within
the uncertainties, all of the baryons can't be closely associated with
galaxies.

There are many lines of evidence for non-baryonic dark matter, the
density of which is many times that of baryonic matter, including
baryonic dark matter. So, at best, finding more baryons in galaxies
would SLIGHTLY reduce the amount of required non-baryonic dark matter,
but not by much.