Density of dark matter along the galaxy I was reading these 2 interesting articles about dark matter inside the solar system:


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*Does Dark Matter affect the motion of the Solar System?

*The Incredible Dark Matter Mystery: Why Astronomers Say it is
Missing in    Action
But I can't figure if:
A) Dark matter doesn't affect the planetary motion because this can't radiate, and thus has large orbits and then is less dense in the inner region of the galaxies and more dense in the outside regions. Or,
B) Dark matter has equal low density across the galaxy (compared with the higher density of baryonic mass inside the solar system), and simply his effects are only important if we sum all the matter in the vast space between stars.
I'm confused because I tend to think that if the dark matter is in a kind of shell, mainly outside the galaxy, this would help to spread the stars far from the galaxy center instead of contain everything in his relative rotation place. I can't find a distribution graph by the way.
Excuse my simple english and thanks in advance for your help!

Edit: I had heard a wrong idea in some documentaries, or I misinterpreted it, about a greater DM density in the outside region of the galaxies. Thanks to @KyleOman for pointing out DM is denser in the center.
 A: Dark matter has a small/negligible influence in the Solar System because there isn't all that much of it in the Solar System, compared to say the mass of the Sun.
The NFW profile is the current default density profile for DM "haloes" (spherical-ish self-gravitating structures, such as the one in which the Milky Way resides). This is a fit to the density as a function of radius for dark matter haloes in cosmological simulations, and in a very broad sense it seems to work pretty well in the real Universe, most of the time (though see below about "cores"). The density is high in the centre, and decreases are $r^{-2}$ for a while, then $r^{-3}$ further out. The formula will give you infinite density at zero radius - clearly this is unphysical, but the point is that the central density rises sharply toward the centre to some high value. Plugging in parameters to the NFW profile for a Milky Way sized galaxy and evaluating the density at $8\,{\rm kpc}$ (distance of the Sun from the centre of the galaxy), I get about $8\times10^{6}\,{\rm M}_\odot\,{\rm kpc^{-3}}$, or about $9\times10^{-19}\,{\rm M}_\odot\,{\rm AU}^{-3}$. The volume of the Solar System is say about $30000\,{\rm AU}^3$, so the DM is outgunned in mass by the Sun by a factor of $4\times10^{13}$. In perhaps more familiar units, my estimate gives $5\times10^{16}\,{\rm kg}$ of DM - compare that with some Solar System bodies and you'll find that it's something along the lines of a medium asteroid. And the DM is diffuse all over the Solar System, so it's even more insignificant than a medium asteroid, gravitationally speaking.
So why is it such a big deal? Because space is big, all that interstellar space has similarly puny densities of DM, but there's so much space that it adds up to a lot of mass - typical estimates say that there should be about $10-100$ times more DM mass in the Milky Way than star and gas mass.
There are other density profiles that are proposed, e.g. Einasto profile, Di Cintio+2014 profile, and a handful of others. Qualitatively they're all fairly similar (except for "cored" profiles, for which I'll point you to a wiki article and, shamelessly, to my own work).
Just to cover all the points in your question, the DM distribution is certainly not a shell outside the galaxy - more like a cloud (denser in the middle) inside which the galaxy lives. And it must be diffuse - it cannot collapse to form dense structures like a DM star or a DM planet (provided something like the standard $\Lambda$CDM theory applies).
Please let me know if you'd like anything clarified, or if you have any followup questions post them and poke me, I'd be happy to have a look :)
