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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?

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  • $\begingroup$ Given that no one knows what dark matter actually is, how do you expect anyone to answer this? $\endgroup$
    – ACuriousMind ♦
    Mar 3 at 13:35
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    $\begingroup$ Just as a friendly suggestion, I think your title can be reworded to better reflect your (quite interesting) question. I think a title like "Why is the ratio of dark matter to normal matter larger in galaxies than the cosmic average?" better captures what you mean. As written, the title makes it sound like you are asking why the cosmic average density of dark matter has one value vs another, which is not really an answerable question. $\endgroup$
    – Andrew
    Mar 3 at 16:05
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    $\begingroup$ It is worth noticing that the intra- and inter-cluster gas may be at least 10 times more ordinary matter than the galaxies. $\endgroup$
    – Anders Sandberg
    Mar 3 at 21:52

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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.

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  • $\begingroup$ Your answer assumes that dark matter comes first, followed by normal matter. How do we know this? Couldn't it be the other way around? $\endgroup$
    – foolishmuse
    Mar 3 at 21:43
  • $\begingroup$ Dark matter doesn't come first it should have been there with normal matter since the first second That being so, why wouldn't it form structures first since normal matter is prevented from doing so by radiation pressure. We know this because structures don't form in model universes unless dark matter starts to form structures before radiation-matter decoupling. @foolishmuse $\endgroup$
    – ProfRob
    Mar 3 at 21:58
  • $\begingroup$ Just using the numbers from your answer, the math actually works out: if we assume there is ~ 5 times more normal matter in galaxies than stellar matter then this would bring down the DM to Normal Matter ratio to about 10 to 1. Then, if half the normal matter is in the intergalactic medium the ratio comes down again to about 5 to 1. $\endgroup$
    – Framazu
    Mar 4 at 8:14
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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.

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  • $\begingroup$ Why can't dark matter clump? If dark matter is made of tiny black holes, they can form clump by gravitating and forming structures. $\endgroup$
    – MatterGauge
    Mar 3 at 18:51
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    $\begingroup$ Dark matter does "clump"; that's why we have galaxies at all. The normal matter falls into overdense regions (clumps) of dark matter. In any case I can't see how this answers the question - it would appear to suggest that the dark to normal matter ratio is smaller in galaxies $\endgroup$
    – ProfRob
    Mar 3 at 20:24
  • $\begingroup$ @ProfRob Further to my question in your answer, are there regions of dark matter in space that do not have galaxies inside them. $\endgroup$
    – foolishmuse
    Mar 3 at 21:50
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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.

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