It's well known that all of our evidence for dark matter (and dark energy, too, but that's not the subject here) at the present time is indirect: It comes from observing the gravitational influence of the hypothetical stuff, not from detecting it "directly" (i.e., using some interaction other than gravitational).
So it's natural to ask whether we can do away with dark matter by positing some modification of the behavior of gravity; I've certainly wondered that myself. And it may very well turn out that the behavior of gravity on large scales does not precisely match the prediction of ordinary general relativity. Nevertheless, I think that, by now, we've accumulated enough data to conclude that the universe cannot be explained solely by modifying gravity; there is ample evidence of gravitational forces pointing in directions where there isn't any (ordinary) "stuff" to create them, leading us to accept the existence of some form of dark matter.
About a year ago I put up a post that explained this point of view, and took aim in particular at the popular framework known as MOND. This led to some good discussion in the comments, and also to a behind-the-scenes email exchange between Rainer Plaga, Stacy McGaugh, and me. It's a bit of old news, but I thought there would still be some interest in our discussion, so (with permission) I'm posting our emails here. Seeing how the sausage is made, as it were. It's a bit of a long read, sorry about that.
Rainer, March 1: Dear Sean, I discussed your recent vigorous defense of CDM your blog with Stacy, and he encouraged me to send you my - absolutely objective ;-) - position. On the one hand I am with you that if Stacy uses terms like "serious fine-tuning problem for LCDM" in his newest paper's abstract (which are then interpreted by science journalists in the way you exhibit), he had to quantitatively compare the expected properties of galaxies under the assumption of LambdaCDM with his data set. If he wants to criticise an idea he has to deal with the idea not with alternatives to it. Alas, he does not do that in this paper. On the other hand I strongly disagree out of principle to require statements like: ?of course we have more than sufficient evidence to conclude that dark matter exists, we?re just trying to understand how it works and what else might be going on.? from anybody. Really Sean, this sounds like a caricature of the holy inquisition to me, "philosophers can speculate as long as they accept that the final truth is already known from the holy scriptures ;-)". Your statement: "Dark matter is real ... there’s no reasonable doubt about the dark matter." is misleading. Stacy and I of course know that dark matter in the form of massive neutrinos does exist beyond reasonable doubt. But that does not answer a crucial question. Crucial questions are: what flattens the rotation curves in galaxies? What creates the third CMB peak? CDM, MOND or something else? In my opinion the final verdict on these questions is not in, yet. Allow me to argue why your top 3 arguments for the existence of CDM do not convince me, perhaps yet. 1. "MOND is ugly": The alternative is not "theory for MOND" vs. GR but "theory for MOND" vs. GR + "theory for CDM particle". The number of exhibited equations then becomes similar. How do you know that TeVeS is uglier than the "theory for CDM particle"? 2. "Clusters require DM anyway" If one could make a case that they require nonbaryonic cold dark matter, I would consider the case settled in favour of CDM. However, the dark matter required for MOND in clusters might be the ca. 40% fraction of baryonic matter that we anyway know is currently missing in clusters (even in LCDM). Do we agree? How can the argument be clinching then? 3. Your strongest argument is the one from the CMB. But still, replacing "MOND" with "CDM", couldn't your statement: "Can some clever theorist tweak things so that there?s a MOND version that actually fits? Probably. Or we could just accept what the data are telling us." be used just as well to comment on the well known problems of CDM to reproduce the detailed properties of galaxies? Wouldn't this be a great topic for another "great debate" a la Shapley/Curtis 1920 between u and Stacy? In that case it turned out both were partly right and wrong, my personal bet: it would be the same this time ;-).
Sean, March 1: Hi Rainer-- Ten years ago, it was perfectly respectable to speculate that there was no such thing as dark matter, just a modification of gravity. (It couldn't have been MOND alone, which was ruled out by clusters, but it could have been some more elaborate modification.) That's no longer true. The Bulltet Cluster and the CMB both provide straightforward evidence that there is gravity pointing in the direction of something other than the ordinary matter. The source for that gravity is "dark matter." It could be simple, like an axion or a thermal relic, or it could be quite baroque, like TeVeS + sprinkles of other dark matter as required, but it's definitely there. If people want to contemplate that there is dark matter and also a modification of gravity, that's fine. If people want to point to features of galaxy/cluster phenomenology and say that these features must be explained, that's absolutely fine. But if people want to cling to the possibility that dark matter doesn't exist, that's not being appropriately cautious, it's just ignoring the data, and it's a disservice to the public to pretend otherwise.
