I love telling the stories of Neptune and Vulcan. Not the Roman gods, the planets that were originally hypothesized to explain the mysterious motions of other planets. Neptune was propsed by Urbain Le Verrier in order to account for deviations from the predicted orbit of Uranus. After it was discovered, he tried to repeat the trick, suggesting a new inner planet, Vulcan, to account for the deviations of the orbit of Mercury. It didn’t work the second time; Einstein’s general relativity, not a new celestial body, was the ultimate explanation.
In other words, Neptune was dark matter, and it was eventually discovered. But for Mercury, the correct explanation was modified gravity.
We’re faced with the same choices today, with galaxies and clusters playing the role of the Solar System. Except that the question has basically been answered, by observations such as the Bullet Cluster. If you modify gravity, it’s fairly straightforward (although harder than you might guess, if you’re careful about it) to change the strength of gravity as a function of distance. So you can mock up “dark matter” by imagining that gravity at very large distances is just a bit stronger than Newton (or Einstein) would have predicted — as long as the hypothetical dark matter is in the same place as the ordinary matter is.
But it’s enormously more difficult to invent a theory of modified gravity in which the direction of the gravitational force points toward some place other than where the ordinary matter is. So the way to rule out the modified-gravity hypothesis is to find a system in which the dark matter and ordinary matter are located in separate places. If you see a gravitational force pointing at something other than the ordinary matter, dark matter remains the only reasonable explanation.
And that’s precisely what the Bullet Cluster gives you. Dark matter that has been dynamically separated from the ordinary matter, and indeed you measure the gravitational force (using weak lensing) and find that it points toward the dark matter, not toward the ordinary matter. So, we had an interesting question — dark matter or modified gravity? — and now we know the answer: dark matter. You might also have modified gravity, but one’s interest begins to wane, and we move on to trying to figure out what the dark matter actually is.
But some people don’t want to give up. A recent paper by Brownstein and Moffat claims to fit the Bullet Cluster using modified gravity rather than dark matter. If that were right, and the theory were in some sense reasonable, it would be an interesting and newsworthy result. So, you might think, the job of any self-respecting cosmologist should be to work carefully through this paper (it’s full of equations) and figure out what’s going on. Right?
I’m not going to bother. The dark matter hypothesis provides a simple and elegant fit to the Bullet Cluster, and for that matter fits a huge variety of other data. That doesn’t mean that it’s been proven within metaphysical certainty; but it does mean that there is a tremendous presumption that it is on the right track. The Bullet Cluster (and for that matter the microwave background) behave just as they should if there is dark matter, and not at all as you would expect if gravity were modified. Any theory of modified gravity must have the feature that essentially all of its predictions are exactly what dark matter would predict. So if you want to convince anyone to read your long and complicated paper arguing in favor of modified gravity, you have a barrier to overcome. These folks aren’t crackpots, but they still face the challenge laid out in the alternative science respectability checklist: “Understand, and make a good-faith effort to confront, the fundamental objections to your claims within established science.” Tell me right up front exactly how your theory explains how a force can point somewhere other than in the direction of its source, and why your theory miraculously reproduces all of the predictions of the dark matter idea (which is, at heart, extraordinarily simple: there is some collisionless non-relativistic particle with a certain density).
And people just don’t do that. They want to believe in modified gravity, and are willing to jump through all sorts of hoops and bend into uncomfortable contortions to make it work. You might say that more mainstream people want to believe in dark matter, and are therefore just as prejudiced. But you’d be laboring under the handicap of being incorrect. Any of us would love to discover a modification of Einstein’s equations, and we talk about it all the time. As a personal preference, I think it would be immeasurably more interesting if cosmological dynamics could be explained by modifying gravity rather than inventing some dumb old particle.
But the data say otherwise. So most of us suck it up and get on with our lives. Don’t get me wrong: I’m happy that some people are continuing to work on a long-shot possibility such as replacing dark matter with modified gravity. But it’s really a long shot at this point. There is a tremendous presumption against it, and you would have to have a correspondingly tremendous theory to get people interested in the possibility. I don’t think it’s worth writing news stories about, in particular: it gives people who don’t have the background to know any better the idea that more or less everything is still up for grabs. But we do learn things and make progress, and at this point it’s completely respectable to say that we’ve learned that dark matter exists. Not what all of us were rooting for, but the universe is notoriously uninterested in adapting its behavior to conform to our wishes.