@Chris Miller

Very interesting post, and one I agree with practically everything on. Whoever your leading astronomer was, I'd agree with him, too, absolutely. The LHC may be able to turn up evidence on any supersymmetry in nature. If nature is supersymmetric, then there exists a lightest supersymmetric particle, into which eventually all other supersymmetric particles will tend to decay. The LSP itself can't decay easily, since the only possible route is into normal standard model particles. If there's an LSP, then, that *is* a dark matter. It may not be abundant enough to solve the dark matter problem either - personally I suspect that an LSP does exist and it won't be abundant enough to be the entire solution - but it's certainly part of the answer.

For the record, neutrinos are technically a dark matter, too, but they definitely cannot be "the" dark matter -- they don't cluster anything like enough to give us the observed distribution of galaxy clusters.

Otherwise, I'd agree with most of what you've said. I'm not at all convinced by MOND itself, since it's an ad-hoc, unmotivated modification of one of Newton's laws - and Milgrom himself is very happy to admit that it's nothing other than phenomenology. It cannot be applied to cosmology and even applying it on cluster scales it dies. However, it fits galaxy rotation curves so well that personally I think it's a sign that there's something interesting going on - even if it turns out to be (chiefly) particulate in origin and happens to be describable by the MOND equations. MOND is not the only modification to gravity being actively studied; a relativistic version of MOND can be described as a "scalar/vector/tensor" theory of gravity (a scalar field - in effect similar in a way to the Higg's, actually - a vector field similar in a way to an electromagnetic field, and a tensor field similar to the gravitational field of general relativity). There are other SVT theories kicking around, and enormous numbers of scalar/tensor theories starting with Brans-Dicke theories back in the 60s. There are also things like "Einstein-aether theories" which attempt to keep practically every allowed symmetry, bimetric theories which are related to SVT theories (or vice-versa, depending which you prefer) and so on. This is definitely an active field of research.

Along those lines, it's also worth noting that galaxies do not live in the Minkowski spacetime of special relativity. Instead, if we assume there is no dark matter, they live in some sort of cylindrical spacetime while if we assume there *is* dark matter they live in a complicated mix of cylindrical and spherical. While the gravitational potentials on the outskirts of a galaxy are certainly small, it's not entirely obvious that they're being defined with respect to the right background spacetime -- we almost universally assume that to be Minkowski. It's still controversial whether this has any impact. Probably not, but in principle we definitely are applying gravity wrongly.

As for dark energy, since I've worked myself on novel ways of dealing with it - in my case, chiefly from the fact that cosmology has gone about things totally the wrong way, assuming the universe to be flat and then adding ripples on, whereas the real universe is actually lumpy and we should instead reconstruct the flat background; the complicated nature of GR means that these approaches, which are equivalent in Newtonian theory, bear no resemblance to one another - I'd balk at saying it's "easier to explain away". Certainly the foundations for declaring it are relatively weak... except that observational support for the standard cosmological paradigm, assuming a dark energy and dark matter, whatever they may be, is staggeringly overwhelming. And attempts to do work like I have leads you either to make approximations that aren't quite valid (as I've done) and find nothing that acts like dark energy, or into a problem that's so complicated as to be practically intractable.

Still, you're totally right and everything absolutely must be considered - and it is being considered. My hunch is that "dark matter" is a combination of a lot of things, including that gravity does not behave as GR on large scales (probably not even on galactic scales, more likely not not on cluster scales, and probably not on cosmological scales), that we're applying it wrongly anyway, that there is at least one supersymmetric particle cluttering up the universe, that there may even be sterile neutrinos, and so forth. Problem is that at the minute the data we have is more than good enough to pin down "dark matter" but as soon as we split that in two or three parts the errors bars go shooting through the roof and you constrain practically nothing.