In DepthAstrophysics

Can dark matter vanquish controversial rival theory?

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Science  27 Jan 2017:
Vol. 355, Issue 6323, pp. 337
DOI: 10.1126/science.355.6323.337

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A long-smoldering feud over the existence of mysterious dark matter is heating up. For decades, a few scientists have argued that dark matter—the stuff thought to make up 85% of the matter in the universe—cannot explain a universal pattern in the motions of spiral galaxies such as our own Milky Way but that a theory called modified Newtonian dynamics (MOND) can. Now, a leading theorist argues that dark matter can explain this pattern after all. Since the 1970s, astronomers have known that the outer stars spiral galaxies circulate faster than they could if the gravity from the visible matter in the galaxy alone were pulling on them, suggesting that some unseen dark matter provides the extra gravity needed to hold them in. But in 1983 one theorist proposed instead tweaking Newton's famous second law of motion, which says an object accelerates in proportion to the force on it. MOND's fix would explain why the out stars can circulate so fast. It explains why a galaxy's behavior can be predicted from the distribution of ordinary matter in it alone. But now a team of theorists say that dark matter can also explain that striking phenomena. The key, they say, is the density of the dark matter "halos" in which galaxies form and a certain gravitational interplay between dark and ordinary matter. If correct, the argument could knock the pegs out from under MOND.