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LIGO/VIRGO instruments are functioning already for quite a while and since then they captured already quite a few instances of gravitational waves which were inferred as products of interactions such as: 1) mergers of black holes; 2) mergers of neutron stars; 3) mergers of black holes with neutron stars; Given that in total the mass of the dark matter is assumed to be much greater than the mass of the regular matter, why gravitational waves, which could be inferred as related to dark matter interactions, are not yet detected by LIGO/VIRGO instruments?

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    $\begingroup$ What dark matter interaction(s) would release as much energy in a short time as those three sources? $\endgroup$
    – Jon Custer
    Commented Jul 1, 2021 at 19:36
  • $\begingroup$ @John Custer Perhaps interactions, involving primordial black holes, which are deemed as made of black matter? $\endgroup$
    – Alex
    Commented Jul 1, 2021 at 20:58
  • $\begingroup$ symmetrymagazine.org/article/… $\endgroup$
    – Alex
    Commented Jul 9, 2021 at 16:44
  • $\begingroup$ newatlas.com/physics/… $\endgroup$
    – Alex
    Commented Jul 11, 2021 at 17:03
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    $\begingroup$ If you have links that help illustrate the question you're asking, it's better edit your question to include them than to post them as a long series of comments. (Also, I would encourage you to be selective about which ones you include, as few people are going to have time to read all of them.) $\endgroup$ Commented Jul 20, 2021 at 17:09

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Black holes and neutron stars are (part of) "dark matter" - in that they are not easily seen/observed. However, if they formed after the big bang then they are only examples of baryonic dark matter. But we think (based on for example the combined analysis of the cosmic microwave background and primordial nucleosynthetic abundances, and large scale structure simulations) that most of the dark matter in the universe needs to be non-baryonic. Thus (non-primordial) black holes and neutron stars are not considered solutions to the dark matter problem.

One thing that non-baryonic matter isn't very good at is clumping together. To make compact objects requires the dissipation of energy and loss of angular momentum, and non-baryonic dark matter can't do that because it doesn't interact electromagnetically with itself or anything else.

But to efficiently produce the power and frequency of gravitational waves that aLIGO detects requires closely separated stellar mass compact objects. Since you just can't form these out of dark matter and the non-baryonic dark matter is thought to be sparsely spread in huge structures surrounding galaxies, then the gravitational wave sources observed are unlikely to have any connection with dark matter.

As an aside, since dark matter does interact gravitationally and gravitational waves are a purely gravitational phenomenon (for merging black holes), then it isn't clear there would be anything different about gravitational waves from a pair of merging black holes made of dark matter in any case.

Edit: I see you mention primordial black holes in the comments. These, if they exist, would be classed as non-baryonic dark matter. They would likely be of much lower mass than the LIGO-detected black holes and so would not be powerful sources of gravitational waves. But, if they were more massive, who is to say that couldn't be the origin of the black holes LIGO is detecting? Although then it is a bit of a coincidence that they also have masses in almost exactly the range predicted by stellar evolution calculations...

A very recent paper by Wang & Zhao (2021) does suggest that the recent black-hole/neutron star candidate mergers could in be the mergers of primordial black holes and that the implied rate of these is still consistent with the upper limits to the possible densities of primordial black holes derived from microlensing experiments. The point is, there is nothing in the gravitational wave data itself that distinguishes primordial black holes from black holes formed after the big bang.

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  • $\begingroup$ Per "New study suggests supermassive black holes could form from dark matter -- ScienceDaily" it is just a hypothesis... sciencedaily.com/releases/2021/02/…. $\endgroup$
    – Alex
    Commented Jul 2, 2021 at 13:19
  • $\begingroup$ Also the notion that neutron stars may consist of dark matter is not quite established... It is not mentioned in the Wikipedia article... en.m.wikipedia.org/wiki/Neutron_star $\endgroup$
    – Alex
    Commented Jul 2, 2021 at 13:48
  • $\begingroup$ @Alex Neutron stars are not "made of dark matter". I am saying they are part of the solution to the dark matter problem. There are in fact two dark matter "problems". One is that most of the normal matter is dark. Neutron stars contribute to this because we cannot observe 99% of the neutron stars in our galaxy - because they are "dark". The other problem is that most matter is non-baryonic. Neutron stars do not contribute to this at all. $\endgroup$
    – ProfRob
    Commented Jul 2, 2021 at 13:56
  • $\begingroup$ No offense, but may be then you could rephrase the first sentence of your answer, which currently says: "Black holes and neutron stars are (part of) "dark matter""... - it is somewhat confusing... $\endgroup$
    – Alex
    Commented Jul 2, 2021 at 14:22
  • $\begingroup$ newatlas.com/physics/… Dark matter or cooling "primordial soup" could create gravitational waves. $\endgroup$
    – Alex
    Commented Jul 3, 2021 at 13:20

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