[In Depth] Can dark matter vanquish controversial rival theory?
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...