A GOOD bit of our universe – 23 per cent of it, to be exact – seems to be made up of stuff we can’t even see. Except, what if it isn’t? The leading cosmological theory says that invisible “dark matter” lurks in the hearts of galaxies, its extra gravity keeping the outermost stars from flying off into the void and generally making clusters of galaxies look the way they do. Nobody knows what this mysterious matter is, but it must be there, because all the visible stars, planets and other bodies do not have enough mass to account for the celestial motions we observe, according to Newtonian gravity. But what if Newton was wrong? It’s a long-standing question whether his law of gravity, supposed to explain everything from falling apples to spinning galaxies, might actually be flawed. That is the claim of a growing number of physicists who support a controversial alternative theory called modified Newtonian dynamics, or MOND.
Gravity: Were Newton and Einstein wrong?