Why is the ratio of dark matter to normal matter larger in galaxies than the cosmic average? There seems to be a discrepancy between the ratio of dark matter to normal matter in the Universe (about 5 to 1 according to $\Lambda$-CDM) and the ratio of the average dark matter halo mass to the mass of the galaxy it contains (somewhere between 50 to 1 and 100 to 1).
As far as I am aware, most of the ordinary matter in the Universe is in galaxies, each of which have a dark matter halo on average at least 50 times as massive as the galaxy itself (see for example Guo, White, Li & Boylan-Kolchin 2010 "How do galaxies populate dark matter haloes?"). In addition, there are many smaller dark matter haloes which do not host galaxies.
Based on this, one would expect that the average ratio of dark matter to normal matter should be at least 50 to 1, yet it is only around 5 to 1. What is the reason for this?
 A: Galaxies are defined by concentrations of dark matter. Normal matter falls into potentials set up by dark matter, becomes dense, undergoes star formation, becomes luminous and we call it a galaxy. It is therefore unsurprising that where we find lots of luminous normal, stellar matter we also find an overdensity of dark matter.
The Guo et al. paper you cite discusses the ratio of dark matter to stellar mass. This is not the same thing as the ratio of dark matter to normal matter because the efficiency of star formation is very low (less than 20% according to Guo et al.) and highly dependent on the dark matter halo mass.
Thus it isn't clear to me that the ratio of dark to normal matter is a lot higher than the average value (indeed, Guo et al. assume it is uniform to calculate the star formation efficiency!)
It is also the case that about half the normal matter in the universe is not concentrated in galaxies at all, it is in the warm-hot intergalactic medium.
A: Normal (baryonic) matter can clump, dark matter can not.
Whether galaxies or planets, these form from much larger gas clouds that condense under the influence of their own gravity. However, conservation of angular momentum implies that efficient clustering requires an efficient mechanism to shed angular momentum. Normal matter does that through friction, which dissipates kinetic energy.
Dark matter has no efficient mechanism to dissipate energy. Actually, there is a mechanism for dark matter to shed angular momentum through gravitational three-body interactions, just like in swing-by maneuvers of space probes angular momentum can be moved from one object to another. But that mechanism is very inefficient, due to the gravitational interaction being so weak. Therefore, the dark matter remains much more puffed up.
What that means is that locally we live in an exceptionally unusual, over-dense region of space (called Earth) which is however embedded in a much larger but much lower-density cushion made of dark matter.
A: If, like you say, there is about 50 times as much DM in galaxies than normal matter, and if the amount of DM in the whole universe is 5 times as big as the amount of normal matter (as observations and the cosmological equations of state tell us) than that can mean only one thing. Namely that there are regions of normal matter containing less DM than 5 times the normal amount of matter.
There are indeed these kinds of regions. Look here, for example. And regions containing other amounts of DM do also exist.
If DM originated in the early universe as primordial black holes (which is possible for holes with smaller mass than the Moon, as these can't be observed by micro lensing effects), or just as particles (which is very unlikely though) it could very well be that normal matter and DM matter interacted to provide different local ratios, while the global ratio stays 5:1.
