Dark matter – that invisible stuff that is supposed to make up some 20% of the Universe – was thought up to explain a puzzling observation. The amount of mass we can see through our telescopes is not enough to keep galaxies from spinning apart. The existence of great quantities of hidden mass would provide the gravitational pull needed to form those galaxies and enable them to rotate in the way that they do.
But not everyone is willing to buy the idea that the Universe is cloaked in “invisible cloth.” An alternate theory, first put forward by Weizmann Institute astrophysicist Prof. Moti Milgrom in 1983, doesn’t require dark matter to explain the phenomenon. Instead, it posits that gravity works differently on the intergalactic scale. With a good tweak to Newton’s formula, the observed Universe falls into place. This is not the violation of a basic law of physics that it might appear: Milgrom points out that gravity works fine in our every-day world, but the formula breaks down at extremes – at the speed of light or in the sub-atomic world of quantum mechanics, for example. So super-galactic scales could be another case in which the rules of gravity simply don’t apply quite as Newton wrote them.
While most are still waiting for the hunt for the mysterious dark matter to yield results, a growing minority of physicists are starting to admit that MOND (modified Newtonian dynamics) could provide a better explanation. Recently, for instance, Prof. Stacy McGaugh of the University of Maryland added fuel to the debate by showing that for galaxies, MOND fits the facts quite reliably – better than dark matter theories.
Even in light of a number of recent studies that lend support to the idea, MOND is considered controversial, and its proponents are often depicted as rebels. Could it be because they keep insisting that the ruling theory, dark matter, has no clothes?