What is the smallest scale at which we have detected "dark matter" (more kinetic energy than would be predicted)?

If you don't mind indulging a follow-up: Would its detection on smaller scales be beneficial at all to advancing our understanding?

(aside: I base this question purely on the notion that our instruments have surely improved a bit since the 1930s.)

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    $\begingroup$ The smallest possible scale of course, not yet measured though, would be to detect DM as particles, i.e. microscopically. Though, to know, what is the smallest macroscopical effect, is interesting. Some researches speculate about DM as a heating mechanism inside stars, which would be a rather small scale. Yet, some other researches looked at DM scattering/accumulation by neutron stars, which would give even smaller scale. $\endgroup$ Dec 13 '13 at 14:45
  • $\begingroup$ Does the motion within our solar system match what's expected by general relativity? If DM is 95% of the mass, surely it would have an effect locally we could measure, right? $\endgroup$
    – Steve Clay
    Dec 13 '13 at 20:14
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    $\begingroup$ the motion of things in our solar system is an effect of many things (GR, gravity from the sun, planets, moons, tidal interactions, etc), but not of dark matter. The reason for that is that the DM has very small density comparing to stellar systems. It has average density comparable to that of the galaxy on galactic scales. Therefore, one cubic parsec contains roughly one solar mass of stellar matter, but also several times solar mass of dark matter. However, stellar mass is concentrated in regions, comparable to solar system, while DM is distributed evenly. $\endgroup$ Dec 13 '13 at 22:50
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    $\begingroup$ Therefore, in the sphere enclosing the solar system there is only $10^{-15}M_\odot$ of dark matter or so, so it is negligible for dynamics. Note also, that DM is not 95% of mass, but rather five times barionic mass and 25% of the total energy of the Universe. The rest is due to dark energy. $\endgroup$ Dec 13 '13 at 22:53

It looks like Segue 2, found by the Keck Observatory, is a candidate for the lightest galaxy, but I don't know if its the smallest one.


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