Nature: Universe has ten times more galaxies than researchers thought
NASA feature: Hubble Reveals Observable Universe Contains 10 Times More Galaxies Than Previously Thought
Headlines sometimes oversimplify. But if this is really true, that there seems to be about ten times more galaxies than had been generally assumed, does this impact other assumptions? Does this imply the universe is ten times more massive, or just that there is less dark matter than was previously thought.
All Conselice et al. (2016) appear to suggest is that when you look at something like the Hubble deep field, there are many faint (and presumably low mass) galaxies that are not seen. This has absolutely no effect on the need for dark matter.
The main results are: (i) as you look back in time, the overall (co-moving) density of galaxies (more massive than a million times that of the Sun) increases. (ii) But the density of more massive galaxies actually decreases. This is consistent with hierarchical merger picture where small galaxies merge to become larger galaxies. This really doesn't have any influence on the need for dark matter.
First, the presence of dark matter is inferred from many different observations. Some of these (e.g. galaxy rotation curves) are not influenced at all if there are lots of extra galaxies.
Second, the "missing" galaxies are at high redshift, not (or not all) in the present day universe, so they cannot significantly affect a calculation of how much normal matter there is in the universe today. Presumably, many of these small galaxies then merge to become larger galaxies and the total mass is conserved.
Third, just because there are lots of them does not mean they contain much mass anyway. The "mass function" (number density as a function of mass) of galaxies goes roughly as $\phi(M) \propto M^{-1}$ at low masses. This means the mass contained in any interval is $$M_{\text{tot}}\propto \int^{M_2}_{M_1} M\phi \ \mathrm{d}M \ = M_2 - M_1$$ So although, low mass galaxies may be ten times more frequent, they are ten times less massive and so don't change the total mass very much. I will need to read the paper more carefully to see if the authors are suggesting that low mass galaxies are much more common in the early universe than was already supposed.
Fourth, primordial nucleosynthesis calculations tell us that only 4 per cent (as a fraction of the critical density) of the energy density of the universe is in the form of baryonic mass. Observations of gravitational lensing, cluster dynamics and the cosmic microwave background tell us that the mass density is actually around 30 per cent of the critical density. Thus most of the dark matter is non-baryonic and cannot be in the form of missing faint galaxies, or any other form of normal baryonic matter.
