I attended a lecture about Dark Matter last night, and found the modelling to be quite troubling. "If it isn't dark, it doesn't matter." Basically, the mass of dark matter exceeds that of ordinary matter by such an amount that to a first approximation, *ordinary matter may be ignored*. Furthermore, not only does dark matter not "interact" with ordinary matter, it does not appear to "interact" with itself, either. https://en.wikipedia.org/wiki/Dark_Matter So far, so good. So what does dark matter consist of? Model 1. Dark matter consists of "atomic" particles all of the same size. In order to avoid weird bosonic behaviors like superfluidity, I would guess that dark matter would consist of fermions. Since these dark matter particles "interact" only through gravity, this model -- except for using general relativity instead of Newtonian gravity -- would be essentially an ideal Maxwellian gas, and would be subject to the barometric formula for pressure. https://en.wikipedia.org/wiki/Barometric_formula Of course, since black holes form due to the extreme curvature of space-time, and since the type of mass that creates this curvature doesn't matter, the center of clusters of dark matter would have to include black holes. Furthermore, once such a black hole formed, any additional mass -- whether dark or not -- would fall into the black hole and add its momentum -- thus adding to the angular momentum of the black hole. (We assume that any charge added to the black hole due to normal matter falling in would be very small relative to the total size & mass of the black hole, so these charge effects can be ignored.) So the center of these dark matter clusters would still be interesting places, having black holes which are at least mildly rotating, but some of which are wildly rotating. With black holes revolving about black holes, there must be some amount of gravitational wave generation going on. Thus, a dark matter cluster would be a more-or-less spherically-symmetrical object with a relatively small overall rotation and a black hole center. The surface of this object would be ill-defined, as the pressure would fall off exponentially according to the barometric formula. The ONLY way for such an object to lose energy would be from gravitational waves. Furthermore, since the particles are small relative to the size of the cluster, there is no mechanism for many of these particles to escape. As a result, this object would not evolve much over the present lifetime of the universe. I would imagine that one could place a limit on the physical size of these dark matter particles (measured in units of mass) by carefully measuring the various statistics (i.e., the classical "thermodynamics") of these dark matter clusters. For example, if the particles were too large, a measurable number of them could statistically "evaporate" and leave the system. Model 2. Same as Model 1, except that the dark matter consists of a multiplicity of different particle types, still "interacting" only by means of gravity. Once again, this is reminiscent of a Maxwellian gas composed of different "partial pressures", and more-or-less classical "thermodynamics" of these ideal Maxwell particles would hold. With enough disparity in the size distribution, it is conceivable that a measurable amount of the smallest size particles could "evaporate", so these types of dark matter clusters might evolve somewhat in the course of 14 billion years. ----- The professor giving the lecture indicated that many of the recent experiments have lowered the limits of the possible interactions between dark and normal matter to such absurd values that the simplest explanation is that there are NO interactions (other than gravity) whatsoever. He personally thinks that these particles may be some sort of "atomic" black holes left over from the formation of the universe. Of course, what keeps these (presumably small) black holes from coalescing (and generating gravitational waves in the process) isn't at all clear. Perhaps they are small enough to have quantum properties that keep them from coalescing -- e.g., fermionic behavior ? These small black holes would also be chargeless, else they could interact with normal matter.