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" ... Adding in just one new ingredient, some form of cold, collisionless dark matter, explains all of these puzzles in one fell swoop. ... "
" ... But as tiny as that number is, it’s only consistent with the massless solution; it doesn’t mandate that the graviton is massless. In fact, if there are quantum couplings to certain other particles, it may turn out that the graviton itself has a rest mass, and if that’s the case, they can clump and cluster together. In large enough numbers, they could even make up part or all of the dark matter in the Universe. Remember: massive, collisionless, minimally self-interacting, and cold are the astrophysical criteria we have on dark matter, so if gravitons are massive — and while we don’t expect them to be, they could be — they could be a novel dark matter candidate. ... "
" ... Definitively and unambiguously, dark matter is required. Specifically, that dark matter has to be cold, collisionless, and invisible to electromagnetic radiation: it cannot be normal matter. If you want to turn up the dial on your skepticism-meter, keep an eye out for contrarian papers that attempt to explain either the cosmic microwave background or the matter power spectrum without dark matter; chances are that they add something in — like a massive neutrino, a sterile neutrino, or an extra field with a specifically tuned coupling — that functions indistinguishably from dark matter. ... "