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Dark matter doesn't interact with electromagnetic radiation, but it, at least, participates in gravitational interactions as known from the discovery of dark matter. But does dark matter exist in a spiral shape around our galaxy?

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Wild guess: Probably not. As I understand it, the arms themselves aren't all that much denser overall than the gaps between them; they're just richer in young bright stars. I wouldn't expect the mechanism that creates and maintains the arms to affect dark matter. –  Keith Thompson Oct 17 '11 at 4:09

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In current cosmological models, the Milky Way resides in a 'halo' of dark matter. Halo is a technical term - in this case, it means a spherically symmetric collection of dark matter. Since dark matter is not self-interacting and does not interact with other matter, it doesn't experience any sort of collisions or friction, and therefore never flattens out into a disk the way normal (baryonic) matter does. So, dark matter does not trace out a disk and does not follow spiral arms.

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Does that mean that spiral galaxies are disk shaped (as opposed to spherical) because of non-gravitational interactions? Wouldn't a spinning mass of gas and dust form a disk even without self-interaction? –  Keith Thompson Oct 17 '11 at 18:54
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A spinning mass only flattens down to a disk because of self-interaction. Galaxies end up disk-shaped because of both gravity and hydrodynamics - the gas (and dust) interact with other gas and dust. They are able to fall under gravity unless supported by some other force. Pressure forces are too weak to prevent them from falling in, but angular momentum is enough to keep a disk formed. Galaxies do generally also have a spherical component consisting of stars because stars rarely interact with each other, so if stars formed in a sphere they stay there. –  keflavich Oct 17 '11 at 21:29
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I would think it wouldn't be spherical. Dark Matter (DM) responds to gravity, and the fact that the normal matter of the galaxy is greater along its equatorial plane, means the normal matter will attract dark matter towards it (as well as vice versa). So there should be a bit more DM in the equatorial plane than a spheroidal model would suggest. Since the DM outweighs the normal matter, it is likely a subtle effect. –  Omega Centauri Oct 25 '11 at 23:02
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Omega - Yes, the disk gravity is moderately greater, but because the dark matter doesn't interact with any of the disk matter, it doesn't collect there. Dark matter particles may accelerate faster towards the disk because of the enhanced gravity, but they'll go right through it and out the other side. –  keflavich Oct 26 '11 at 0:42

Disclaimer: I don't do dark matter, I don't work with anyone whose does and I haven't read enough papers to make an difference. I have seen a colloquium and a couple of seminars by people who do do dark matter.

Take what follows with that in mind.


Dark matter either not having been observed or having served up exactly one unconfirmed observation there is no widely agreed theory of its behavior yet.

There are models of cosmological evolution that posit certain traits for the dark matter and when run forward from the presumed conditions of the early universe give something that resembles the universe we see today.

In those models, the dark matter distribution is roughly spherical and rather larger than the luminous portions of the galaxy.

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The 'spiral arms' do not contain a lot more matter than the 'not spiral arms'. Rather we pick these out as they glow bright with new star formation. The density distribution is fairly constant throughtout the disc.

According to the latest modelling the dark matter component is a spherical shape on small scales but forms threads between conglomerations centered on galaxy clusters. Given dark matter forms 5x the mass of light matter it makes sense to reverse this and say light matter congregates within dark matter structures.

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The main way I know of for studying dark matter distribution (I actually just wrote a grant application for research relating to it! :P) is gravitational lensing. Depending on the geometries involved, one can fairly precisely model dark matter distributions by looking at distortion of "nearby" (as far as arc-seconds on the sky, not actual physical distance in space) galaxies by weak gravitation lensing. A nice example (with a pretty picture) is available here.

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Since dark matter is 96% of the matter in the universe, the question is complicated. In fact we're distributed around dark matter, if anything. That's a bit in general, and is covered a bit in analysis of Millennium I and II (which are incredible search for Cosmography of the Universe and stick with the twenty minutes video). Schneider More appropriately there has bee work to constrain the model of the exact subject of your question [Vera-Ciro][2].

The shortest version is no, the shape of dark matter halos is for the most part elliptical, with the eventual relaxation into a spherical halo as time runs.

Disclaimer: this is not my particular field, but I've been reading recent papers on it because it provides quantitative data for something I'm thinking about.

cheers

[2]: http://arxiv.org/abs/1304.4646 "

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