Dark matter is our best explanation for why galaxies stay together when they don't seem massive enough to keep up gravitational attraction. But now a largely-dismissed alternative theory has some actual proof backing it up. This could get complicated.
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Dark matter, which calls for the existence of massive particles (or some other form of matter) that are not detectable using conventional means but do have gravitational effects, is generally accepted by physicists, even if its apparent inelegance is a turn-off to some educated laypeople. (I won't rehash the arguments for the existence of dark matter, because Dr. Dave Goldberg already did it way better than I ever could.) Still, there have been attempts to advance alternative theories to the dark matter model, and the best-known is probably Modified Newtonian Dynamics, or MOND.
Back in 1983, Israeli physicist Moti Milgrom hypothesized that there was a missing feature in our understanding of Newtonian dynamics. Basically, at the very low acceleration scales of stars in their galaxies, the gravitational force actually increases to the levels needed to hold galaxies together. The idea was that all previous work on Newtonian acceleration dealt with relatively high rates of acceleration, but over cosmic distances acceleration is a completely different beast and modified laws might hold sway.
MOND gained some supporters when it was able to correctly predict the observed rotation rates of galaxies, but this support eroded when analysis of the Cosmic Microwave Background - the leftover energy echo of the Big Bang - provided some extremely convincing confirmation of dark matter theory.
University of Maryland physicist Stacy McGaugh hasn't given up on MOND, however, and now he believes he's found some crucial evidence for it after all. He considered the rotation speeds of gas-rich galaxies, finding that more massive galaxies rotate proportionally faster than their less massive counterparts, all in keeping with the predictions of MOND. The dark matter model, on the other hand, was far more error-prone when it came to predicting these particular rotation speeds.
So what does this mean? It definitely doesn't mean we have to throw out dark matter - this is just one very specific result that suggests - but doesn't prove - that in one particular circumstance, MOND seems to describe things better than the dark matter model can. That's an important result, but it's hardly enough to overturn the experimental evidence we've already gleaned from the cosmic microwave background and other areas that clearly support dark matter over MOND.
But this might just be telling us our understanding of dark matter is more incomplete than we already thought it was, and that some MOND-like interactions do exist. Indeed, even McGaugh says this result should probably be seen in the context of dark matter:
"I think it's telling us something about nature. Maybe it's telling us something about the nature of dark matter."
One possibility is that dark and normal matter can interact in ways that skew the accepted cosmic value for dark matter, which is that about 5/6 of all matter is dark. In the gas-rich galaxies, this value might be significantly off, which explains why the dark matter models are presently constructed were unable to describe the galaxies as well as MOND could. Yale cosmologist Priyamvada Natarajan points out that there are still plenty of places where MOND simply does not work, but McGaugh's work could well point to some as yet unknown phenomena:
"It reveals there are physical processes we don't understand. We don't fully understand how baryons [ordinary matter] and dark matter interplay. I see this as a challenge. It points the way forward."