Professor Siegel, I'm a fan of yours and I dearly love reading your work. Concerning dark matter, I hate to sound like a know-nothing crackpot - and I know I probably am, for I don't have a fraction of your knowledge and education - but I'd like to offer a different possible explanation for dark matter, because I suspect we might be stuck with the wrong paradigm, that the fabric of space itself may not be uniform in all locations.
I tried to explain it better in the article linked to below - here's the pertinent paragraph:
"...[if] quantum foam does exist but is not equal throughout every cubic millimeter of the universe, then that opens up the very real possibility that the fabric of space itself is comprised of different consistencies (or frequency, intensity, or whatever — pick your descriptor) depending on whether it’s within a solar system, or within a galaxy, or within a galactic cluster, or outside any of those. If the consistencies are not equal, then different regions of the universe may well be likened to oil and water, thus leading to a geometry precisely like that described in the Sloan Digital Sky Survey above."