DARK MATTER: IS IT TIME WE GAVE UP LOOKING FOR IT?

After decades of looking for dark matter and coming up short, some researchers say we should take the possibility of a new theory of gravity more seriously

- MARCUS CHOWN

Two cosmic anomalies tell us that something big is missing from our model of the Universe. First, stars in the outer regions of a typical galaxy are orbiting the centre too fast for the galaxy's gravity to hold onto them. By rights, they should fly off into intergalactic space.

The second anomaly is that you are reading these words that is, galaxies like the Milky Way, and therefore you, exist. According to the standard picture of galaxy formation, regions of the cooling debris of the Big Bang that were slightly denser than average would have had slightly stronger gravity and pulled in material faster, enhancing their gravity so they pulled in matter even faster, and so on. But this process akin to the rich getting ever richer - could not have built galaxies as big as our Milky Way in the 13.8 billion years that the Universe has existed.

Confronted with these anomalies, most astronomers postulated that the Universe contains about five times as much invisible matter as visible stars and galaxies. It is the extra gravity of such 'dark matter', they claim, that holds onto stars in galaxies and sped up galaxy formation. However, an equally logical possibility is that, on cosmic scales, gravity is stronger than Newton would have predicted.