In other words, it could be another source of gravity, but we're still not sure. He's simply discounting any 'theory' (by which he probably means hypothesis) that doesn't 'make sense', but we still don't have any evidence to distinguish between these competing hypotheses. He provides no reason for discounting the other hypotheses, he simply declares them 'insensible', nor does he bother to even mention what those other theories are.
I'm not Sean, so I don't know what he meant by that. But by theories that make sense he probably means ones:
a) not blatantly made up by crackpots who don't know the observations they need to explain, or
b) those without unsavoury properties (if a theory predicts tachyons or
ghosts you might disfavour it for example) [think that's an appropriate link, ages since I looked into ghosts to be honest]
c) those too complex - have too many free parameters, bells and whistles attached - it's better to minimally modify your model to fit observations
You don't actually need evidence that doesn't fit a model to choose between models. You can simply chose the simpler and stronger one as your currently favoured.
Unless he is using a very broad use of the term 'Gravity'. Gravity is sometimes used as a catch-all for any apparent force that is proportional to the mass of the affected object. This causes accelerations to be independent of mass. One such use is to define the centrifugal force term of a rotating reference frame as a gravity field. However, this seems unlikely, as nearly half of the article is dedicated to the problems inherent in using overly broad, and commonly misunderstood, terms like 'energy' and 'dimensions'.
He's using it in a cosmological context. He therefore either means general relativity or a direct substitute for it.
arthwollipot - it'd be an understatement to say it's considered unlikely. There's very strong constraints on that 4% figure from the start of the thread. That's saying that only a small proportion of the total matter can be in the form of everyday atoms like stars or planets. We know that it's something we've not seen before that has essentially no interaction with light - it can't be merely everyday stuff that's not emitting light. It's got to be something else.
sol invictus is as correct as ever. 'It's almost certainly out there, and actually I don't find its existence particularly surprising.' particularly is something I agree with. Why should you find such matter surprising when we've already found matter that behaves in much the same way, and was similarly motivated by observations that didn't match expectations (I'm talking about neutrinos)? We've made the jump from wanting to preserve one important part of physics (conservation of momentum and energy) to supposing there was an undetected particle before, and been right. Is it really so dangerous to do the same for another important part of physics (gravity), at least until there's an explanation that fits the facts better?
