# Does Dragonfly 44 'shed any light' on the theories of dark matter?

First I'll apologize for my pun; I just couldn't resist.

But seriously, last year astronomers observed the dim galaxy, Dragonfly 44, and a recent article in Science reports that the content of dark matter in the group of stars observed is about 99.9% which I believe is a first observation of dark matter of concentration well beyond the more typical 23% observed in the rest of the Universe.

I know that physicists don't really understand quite yet what dark matter is, and I admit I'm not fully briefed on the details of any theories, but does this new discovery discount any present theories or put more emphasis on others as to what comprises dark matter?

• Note that dark matter $\neq$ dark energy. Dark energy is the thing that takes up $\sim 70\%$ of the energy content of the universe, and is (at least on large scales?) homogeneously distributed. – Danu Aug 28 '16 at 21:31
• If these objects turn out to be plentiful would it change the estimate of the percentage of DM? – Keith McClary Aug 29 '16 at 1:14
• @Keith Ask the question. – Bob Bee Aug 29 '16 at 2:16
• I supposed they have already ruled out a supermassive black hole at the center keeping the stars together. Reading the paper I see " There is also no evidence for radial variation in the velocity dispersion.." which is I assume very unusual and thus indicative of the properties of dark matter (maybe)? – ja72 Aug 29 '16 at 3:21
• @ja72 "very unusual and thus indicative of the properties of dark matter (maybe)" Actually it's typical of large galaxies that outside a certain distance from the center the orbital velocity distribution remains approximately constant, but that is attributed to the presence of a dark matter halo as the visible mass certainly can't explain it. Nor should this be surprising because even in normal galaxies there is much more dark matter than baryonic matter, so the effect of losing most of the normal stuff can be expected to be small. – dmckee --- ex-moderator kitten Aug 29 '16 at 17:01

Part of a galaxy, at least in the definition I'm used to, is its dark matter halo. While the halo is on average quite massive, about $\frac{5}{6}$ of the total mass of the galaxy (the other $\frac{1}{6}$ being made up of stars and gas; this comes from the cosmic average abundances, being careful to note that dark energy doesn't come into the mass budget for a galaxy), it is also quite extended, so the density doesn't get all that high. There seems to be a relationship between the mass of the halo and the (stellar) mass of the galaxy that ends up forming inside it, though with a fair bit of scatter. There is also a correlation between the (stellar) mass of a galaxy and its size, again with quite a bit of scatter.