To me, the dark matter hypothesis seems to be rather like Ptolemy's epicycles, Descartes invisible Vortices or the aether: a fudge. One question is: if so-called dark matter only feels gravity, then what keeps it from collapsing in on itself? The collapse under gravity of ordinary matter can be arrested because, to give one example, as matter is squeezed, temperatures rise, fusion begins, and an outward pressure appears (the exchange of momentum is due to the electromagnetic, EM, force) and a balanced system, a star, is formed. This could never happen with dark matter (DM) since it does not feel the EM force and so it can't be held up by pressure (but despite this, they say it can still lose kinetic energy, and collapse inward, via three-particle collisions). To allow general relativity to predict galaxy rotation curves correctly huge amounts of dark matter must be assumed to remain in an uncollapsed state in a huge sphere (halo) around a galaxy's visible matter. The gravitation forces on this halo would be ten times that due to the baryonic matter and the halo is not spinning to provide inertially-supportive forces (no flattening) so the dark matter should have collapsed quickly. To be held out there, gravity must be assumed to be balanced by some outward force.
This force can't be assumed to be the EM force (pressure) or the strong force because DM can't possibly feel them (if it did, experiments would have seen it) and the weak force is limited to very short ranges of ~10^-17 metres.
What keeps the proposed DM in an uncollapsed state? The physics does not exist to do it. People often refer to MiHsC as "exotic" physics, but it seems exotic physics is unavoidable. Either invent a new kind of invisible (dark) matter and new physics to go with it, or (neater & simpler) keep matter unchanged and modify only the physics. MiHsC is an example of the latter approach, and models galaxy rotation like this.
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