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?
Best Answer
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.