Known properties and constraints of dark matter

[SlowThinker]

I'm wondering what properties of Dark Matter are known for certain. Such as:

• How much does it collide with itself, and with ordinary matter? What does $\sigma/m\le 7cm^2/g$ mean (and why is the estimate rising in more recent studies)? How does it compare with ordinary matter, e.g. liquid water or galaxy filled with stars?
• Is there any evidence that DM does interact with ordinary matter via weak interaction? Is lack of DM particles in LHC a constraint on this?
• Can it have more than 1 component?
• How does its density distribution compare to ordinary matter? How much spherical are galactic halos, and how much bigger are they compared to ordinary matter?
• How much of it is out there? Is the 5:1 ratio to ordinary matter still the best estimate?
• If it's made of massive dark objects, what mass ranges are excluded?
• If it's made of particles, what mass ranges are excluded?
• Is anything known about its temperature?
• How does its amount change since the Big Bang?

I'd appreciate layman answers
I can try to read some publications, but the amount of literature is overwhelming and most of it seems to be outdated.

 

[Orodruin]

I'm wondering what properties of Dark Matter are known for certain. Such as:
How much does it collide with itself, and with ordinary matter? What does $\sigma/m\le 7cm^2/g$ mean (and why is the estimate rising in more recent studies)? How does it compare with ordinary matter, e.g. liquid water or galaxy filled with stars?

We do not know. There are some studies that suggest that self-interacting dark matter may be preferable in terms of obtaining the correct halo density structure, but it is really not conclusive. The $\sigma$ is the size of the interaction cross section, which is essentially how likely it is that two dark matter particles collide. The $m$ is the mass of the dark matter particle. Typical ranges of the mass is in the GeV scale, which would mean that 1 cm²/g would correspond to a cross section in the range of $10^{-28}$ m² (in very rough terms, this is the area within which the particles have to approach each other to interact).

Is there any evidence that DM does interact with ordinary matter via weak interaction? Is lack of DM particles in LHC a constraint on this?

There is no direct evidence of this. Lack of DM particles in LHC constrains such dark matter with masses below LHC energies.

Can it have more than 1 component?

Sure, there is nothing that prohibits this. There could be an entire dark sector with many types of particles.

How does its density distribution compare to ordinary matter? How much spherical are galactic halos, and how much bigger are they compared to ordinary matter?

Dark matter halos are quite spherical but can also show some granularity according to simulations. They are quite a lot bigger than the galaxies that inhabit them.

How much of it is out there? Is the 5:1 ratio to ordinary matter still the best estimate?

Yes, this does not change with time unless dark matter decays or annihilates. In many models dark matter can meet and annihilate to standard model particles, but this is a slow process. If dark matter decayed very fast there would be none left now, which places constraints on the dark matter lifetime.

If it's made of massive dark objects, what mass ranges are excluded?

You may be thinking of what is called MAssive Compact Halo Objects (MACHOs). There are quite strong constraints on such objects from microlensing studies to the extent that it is doubtful that they can constitute the dark matter.

If it's made of particles, what mass ranges are excluded?

You really cannot rule out any mass range. It depends on the other properties of the dark matter such as its interaction cross sections. The typical exclusion plot you will see will be in the mass vs cross section plane.

Is anything known about its temperature?

It is doubtful that it is meaningful to talk about a temperature in the same sense that you can talk about the temperature of the CMB.

How does its amount change since the Big Bang?

See above. It will not change unless it annihilates or decays.

 

[Sanborn Chase]

We might ask if it exists in phases as does ordinary matter.

 

[SlowThinker]

The $\sigma$ is the size of the interaction cross section, which is essentially how likely it is that two dark matter particles collide. The $m$ is the mass of the dark matter particle. Typical ranges of the mass is in the GeV scale, which would mean that 1 cm²/g would correspond to a cross section in the range of $10^{-28}$ m² (in very rough terms, this is the area within which the particles have to approach each other to interact).

So that is not very different from ordinary matter? How is that possible? Is it only DM-SM interaction that is negligible? Or are the bounds inaccurate to several orders of magnitude? My numbers come from http://cosmology.lbl.gov/talks/Harvey_13.pdf page 8.

Yes, this does not change with time unless dark matter decays or annihilates. In many models dark matter can meet and annihilate to standard model particles, but this is a slow process. If dark matter decayed very fast there would be none left now, which places constraints on the dark matter lifetime.
...
See above. It will not change unless it annihilates or decays.

This article suggests that the amount of DM increases over time, or perhaps it takes a long time to form DM halos around galaxies. I am unable to judge the article's reliability.

You may be thinking of what is called MAssive Compact Halo Objects (MACHOs). There are quite strong constraints on such objects from microlensing studies to the extent that it is doubtful that they can constitute the dark matter.

This article from year 2000 says that MACHOs can't make 100% of DM, but, on the other hand, the amount of microlensing is several times higher than expected from stars alone, estimating that MACHOs make about 20% of DM. Is there any new development?

It is doubtful that it is meaningful to talk about a temperature in the same sense that you can talk about the temperature of the CMB.

There is average kinetic energy. Is that supposed to be about equal to CMB temperature? Is there any experiment that could measure the average kinetic energy of DM particles? There should be constraints derived from the size of galactic halos. Too much energy, DM flies away; too little and the halos would be too small.

 

[SlowThinker]

We might ask if it exists in phases as does ordinary matter.

I'm afraid that cannot be answered with gravitational observations, which is pretty much the only thing we have.

 

[Chronos]

We assume dark matter is particulate largely because all other forms of matter appear to be particulate. The allowable mass range for MACHO's is severely constrained by microlensing surveys. No more than about 25% of dark matter can exist in the form of MACHO's of a mass exceeding about .0000001 [10-7] solar masses within the galactic halo - re; https://arxiv.org/abs/astro-ph/9803082, EROS and MACHO Combined Limits on Planetary Mass Dark Matter in the Galactic Halo.

 

[Sanborn Chase]

I'm afraid that cannot be answered with gravitational observations, which is pretty much the only thing we have.

Don't we have some of that gravity beneath our feet? Are we searching right here on Earth with as much vigor as deep space?

 

[SlowThinker]

Don't we have some of that gravity beneath our feet? Are we searching right here on Earth with as much vigor as deep space?

Because gravity is so weak, it's really hard to find or measure small things with it. And if you can't hold them in place for a long time, it's pretty much impossible.
I'm wondering if we can use knowledge of nuclear reactions to estimate the mass of the Sun, and compare it to its gravitational mass, to see if he Sun perhaps has some dark matter in its core.

 

[Sanborn Chase]

I fell and broke my hip recently; I don't think gravity is weak at all.
Your idea about the sun seems good, but what about the mass of the Earth?

 

[Vanadium 50]

I fell and broke my hip recently; I don't think gravity is weak at all.

But it took a whole planet to do it,.

 

[Chronos]

I have a refrigerator magnet that laughs at the strength of Earth's gravity.

 

[fresh_42]

I have a refrigerator magnet that laughs at the strength of Earth's gravity.

I have some old ones which stopped laughing and began fighting ...