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Newton’s Theory of Gravity explains most large-scale events fairly well. ... However, the theory is not foolproof. Einstein’s theories of general and special relativity, for example, explained data that Newton’s theory couldn’t. Scientists still use Newton’s theory because it works in the overwhelming majority of cases and has much simpler equations. Dark matter was proposed as a way to reconcile Newtonian physics with the data. But what if, instead of reconciliation, a modified theory is needed.... Mordehai Milgrom...developed a theory of gravity (called Modified Newtonian Dynamics or “Mond” for short) in 1982 that postulates gravity functions differently when it becomes...

A detailed analysis of eight dwarf galaxies that orbit the Milky Way indicates that their orbital behaviour can be explained more accurately with Modified Newtonian Dynamics (MOND) than by the rival, but more widely accepted, theory of dark matter. The results were presented by Garry Angus, of the University of St Andrews, at the RAS National Astronomy Meeting in Belfast on Wednesday 2nd April. 'MOND was first suggested to account for things that we see in the distant universe. This is the first detailed study in which we've been able to test out the theory on something close to home....

Dark matter and normal matter have been wrenched apart by the tremendous collision of two large clusters of galaxies. The discovery, using NASA's Chandra X-ray Observatory and other telescopes, gives direct evidence for the existence of dark matter. "This is the most energetic cosmic event, besides the Big Bang, which we know about," said team member Maxim Markevitch of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. These observations provide the strongest evidence yet that most of the matter in the universe is dark. Despite considerable evidence for dark matter, some scientists have proposed alternative theories for gravity where it...

Is string theory the answer to the last big questions in physics, or a dead end? While some physicists believe that string theory could lead to a unifying 'theory of everything,' detractors say that string theory is sloppy and founded on unwarranted assumptions. One new book calls string theory 'not even wrong.' In this hour of Science Friday, we'll look at the pros and con of string theory, with physicists Lee Smolin and Brian Greene.

If Newton saw today's astronomical evidence, would he come up with a different law of gravity? A growing number of people think so, says Marcus Chown There's something wrong with our understanding of spiral galaxies such as our own Milky Way. The stars in their outer parts are being whirled around far too fast. Like children on a speeded-up roundabout, they should be flung into intergalactic space. To explain why this does not happen, astronomers have been forced to propose that the visible stars and nebulae are supplemented by at least 10 times more invisible stuff. The gravity of this...