According to people in the UCSC Physics Department, it is consistent with current observations that dark matter interacts only gravitationally.  I'll ask again when I next go to the Physics Colloquium, in case that claim is now out of date. The density of dark matter can be calculated from galactic velocity curves.  For the Milky Way, the velocity of orbiting visible matter is pretty much constant at 225 km/s, except near the center, where it decreases.  From m v^2 / r = G M(r) m / r^2, we get for the enclosed mass within radius r, M(r) = (v^2 / G) r. The mass density is ρ(r) = (dM(r)/dr) / (4πr^2) = (v^2 / 4πG) r^-2      = (6.0e19 kg/m) r^-2 At our location 28000 ly = 2.8e20 m from the galactic center, ρ = 7.7e-22 kg/m^3 = 0.43 (GeV/c^2) / cm^3. The gravitational acceleration varies as 1/r, so the potential φ varies as log r, and the Laplacian of φ varies as 1/r^2, just like ρ, so that ρ and φ satisfy the Poisson equation.  Also, for a suitable choice of m/T, ρ = exp(-mφ/kT), so that ρ and φ satisfy the Boltzmann equation.  Thus the 1/r^2 density implies that the dark matter has equilibrated. Now here's a question for which I don't know the answer.  If dark matter equilibrates with itself, it should presumably also equilibrate with ordinary matter.  But ordinary matter can dissipate energy and coalesce.  So why can't dark matter do so too using ordinary matter as an intermediate? There's no evidence as to whether dark matter is a boson or fermion.  It is certainly something stable.  My guess is that there is a new, yet unknown, conserved quantum number; call it D for darkness.  Then D=0 for ordinary matter, and D=1 (and maybe also D=-1) for dark matter.  The lowest mass D=1 particle is then stable against decay to a D=0 state.   --  Gene From: Warren D Smith <warren.wds@gmail.com> To: math-fun@mailman.xmission.com Sent: Sunday, March 29, 2015 5:04 PM Subject: Re: [math-fun] Dark matter exists? To Meeker: dark matter could not be gravity-only because then it would be too-little-interacting to achieve that paper's claimed 0.1 cm^2/gram cross section lower bound.  (If believed.) But yes, it definitely gravitates, that's only reason why it is claimed we know it is there. To Diffie: GOM (gravity-only-matter) could not clump, far as I can see. E.g. two particles would head toward each other, fly off, hyperbolic orbit, no energy lost, no capture possible. However... if the matter ALSO interacts via EM forces, then capture is possible, for example 2 planets collide, lots of heat photons emitted, result is a bigger planet, or in the Earth's case planet+moon, plus lost energy. GOM actually could lose energy via gravity waves (gravitational bremstrahlung) but that'd be an incredibly weak loss mechanism, way insufficient to achieve clumping given the present young age of the universe. The only reason normal matter has clumped into nuclei, atoms, molecules, planets, stars is, EM forces allow energy loss to far away via photons. Dark matter by definition fails to couple to photons, ergo no clumping. Furthermore, blow off that theoretical jive and just look at the facts ma'am: the distribution of dark matter is tolerably well known in various galaxies, etc, and it evidently did not clump, did not lose energy to form disk not blob, etc. Somebody else: Is dark matter fermions or bosons? Well, I would presume fermions since they are less easily created & destroyed than bosons, and DM seems to be pretty permanent stuff.  But given how little is known about DM, it's hard to be sure.  In fact, I am unsure DM even exists at all, I think it might be there is no DM and it is just that we have a wrong theoretical picture of the laws of gravity.  (There is a small but fairly determined minority who hold that view.) In fact I have a perpetually incomplete paper I am writing about such a possible improved(?) theory of gravity which I've invented as a counterweight to DM-mania. -- Warren D. Smith http://RangeVoting.org <-- add your endorsement (by clicking "endorse" as 1st step)