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)