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What methods do we use to detect Dark Matter?

If I understand correctly, due to lack of electromagnetic interaction it should be able to phase through normal matter nearly like through void - since the structure of matter outside the (relatively tiny) nuclei is based upon electromagnetic interactions between atoms and within atoms. It also doesn't exhibit the Strong forces which bind the nuclei, making them somewhat "transparent" as well.

It acts gravitationally, but that's difficult to detect if there are no observable objects affected by the gravity there, and it exhibits the Weak interactions, which only very lightly affect nuclei of atoms.

So, what do dark matter detectors measure directly, deriving density of dark matter from the measurement?

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  • $\begingroup$ The word "dark" means that it is not detected. "Dark" is what astronomers/cosmologists call anything that they can not see, but whose existence they infer from other observations. $\endgroup$ – Solomon Slow Dec 17 '15 at 17:47
  • $\begingroup$ @jameslarge: last sentence of the question. WHAT other observations? Specifically on small distance scales (solar system radius ~ touch distance), as we've mapped galaxy-sized and larger concentrations quite accurately. $\endgroup$ – SF. Dec 17 '15 at 18:10
  • $\begingroup$ See Martin's answer (below). Especially, the part about rotational speeds of objects in other galaxies. $\endgroup$ – Solomon Slow Dec 17 '15 at 18:54
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As of now, there is no dark matter detector that directly measures dark matter densities. You point out the main problem: If dark matter doesn't interact at all except through gravitation, how can we measure it? I don't think we could. However, we don't know that dark matter doesn't interact at all, simply because we don't know what dark matter really is. It definitely doesn't interact much, but not at all? There are many proposals as to what dark matter will ultimately turn out to be and most of these proposals make predictions one can actually measure.

Hence, there are many examples for proposed measurements of dark matter such as Xenon1T which are based on various proposals of what dark matter actually is. Xenon1T for example supposes that dark matter consists of a new type of particles that do interact, just very very weakly (way more weakly than neutrinos. They are therefore called "weakly interacting particles" also called "WIMPs"). I discussed a few different approaches at directly measuring dark matter here. As of now, none of them has provided conclusive evidence for having measured "dark matter".

How do we know the density of dark matter then? Well, it's always indirectly. Take a galaxy: You can measure the rotation speed of objects in a galaxy and from the speed of the objects at various distances from the galactic centre, you can infer the mass density. Now you substract what you CAN see, i.e. usual baryonic mass and you notice that this is not enough. The rest is then just dubbed "dark matter", because we obviously can't see it and don't know what it is...

And this is just one way. There are a many other areas where there must be more matter than people can "see" (see Wikipedia for more areas).

As pointed out in the comments, there was some flury of news about our solar system and even all planets having "hair", very dense filaments of dark matter. This idea (see this paper) however is not the result of direct measurements, but it follows from numerical and analytical calculations.

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  • $\begingroup$ It's okay on galactic scale where there's both lots of the matter (and as result it exhibits a lot of gravity) and a lot of objects affected by it. But how can we detect features like "hair around stars and planets"?? $\endgroup$ – SF. Dec 17 '15 at 15:25
  • $\begingroup$ None of this has been measured. The paper this refers to (arxiv.org/abs/1507.07009) is based on numerical simulations - in other words: These are just predictions. The predictions are interesting though, because if there would be a heap of dark matter close by, we could maybe send probes there and find something new to help us explain the thing. $\endgroup$ – Martin Dec 17 '15 at 15:45
  • $\begingroup$ Yes, except the probe could be sitting squat in the middle of a super-dense cloud of dark matter and still not detect anything if we don't have any detectors that could directly capture it or otherwise measure it at hand's reach. $\endgroup$ – SF. Dec 17 '15 at 16:27
  • $\begingroup$ @SF: Yes, and I'm telling you that as of now, we don't have any such detectors. We have never directly detected dark matter, yet. I linked to a page where I gave a small overview about various approaches to directly measure dark matter (based on various theoretical proposals), but none of them has unambiguously detected anything, yet. $\endgroup$ – Martin Dec 17 '15 at 16:36
  • $\begingroup$ Oh. I misinterpreted your "None of this has been measured" believing it wasn't measured because we never got our instruments near any significant concentration, not that we don't have any instruments that we're fairly sure should work. $\endgroup$ – SF. Dec 17 '15 at 16:59
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There is no direct way to measure dark matter. In SUSY and other theories, dark matter particles can interact with baryonic matter [wiki] https://en.wikipedia.org/wiki/Supersymmetry#Dark_matter) but this has no consequence at cosmological scales. The only way to measure dark matter is by analyzing the properties of baryonic matter and to see the velocity dispersions of galaxies by presuming that the baryonic matter is "observable". Dark matter particles can be cold, hot, or warm.

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  • $\begingroup$ What you are saying is that we need an eplanation to account for gravitational anomalies and that dark matter is a way to explain the anomaly. And the way we measure dark matter is by th amount of the anomly. Kinda circular reasoning, no? $\endgroup$ – Peter R Dec 17 '15 at 20:33
  • $\begingroup$ Indeed, we measure dark matter indirectly. However, the actual standard model has some problems, one of them is CP problem and the axions were introduced to solve this problem. Axions might count for a cold(non-relativistic) dark matter particles. link $\endgroup$ – Mikey Mike Dec 18 '15 at 10:10
  • $\begingroup$ Standard model has other problems too like dealing with gravity. The assumption is being made that there must be some unseen mass without any independent verification. If no independent verification can be done, would dark matter still be accepted as a viable explanation? $\endgroup$ – Peter R Dec 18 '15 at 16:02

protected by Qmechanic Dec 17 '15 at 20:45

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