According to our tried-and-tested laws of gravity, galaxies do not have nearly enough visible matter to stop themselves flying apart as they rotate. This is the main reason why physicists think they must contain an extra component of "dark matter", which as the name suggests has remained hidden despite our best attempts to observe it directly. This could be because its interactions with ordinary matter are so weak that it has passed under the detection threshold of all current laboratory experiments here on Earth. On the other hand, it could interact via some long-range force different to the four forces that appear in our established Standard Model, meaning that it would only ever be revealed indirectly by studying large objects such as galaxies.

Glennys Farrar and Rachel Rosen of New York University in the US think that evidence for such a "fifth force" could be found by studying a collision between two individual galaxy clusters some three billion light-years away. This collision, which has been collectively nicknamed the Bullet Cluster, consists of a smaller "bullet" galaxy cluster, which has passed through an exceptionally large galaxy cluster.

The trick to their reasoning is to place plausible limitations on what the initial approach speed and mass distribution of the bullet could have been, given what astronomers already know about the dynamics of other galaxy clusters. Then, by recording how fast the bullet is moving away from the larger cluster, they would be able to deduce whether the acceleration is the value that would be produced by gravity alone acting on dark matter.

Farrar and Rosen have already tested their theory using x-ray data taken by the Chandra satellite, which suggest the speed of the "shock front" of the bullet to be 4740 km/s. This speed is too high to have been produced by gravity alone, implying that gravity acting on the dark matter in the Bullet Cluster is being augmented by a fifth force between 0.4 and 1.2 times as strong. Such a fifth force, if it does exist, would be proof that the Standard Model is incomplete -- possibly requiring extensions such as supersymmetry that include additional force-carrying particles.

However, the physicists now think that the speed of the bullet given by the Chandra data may not be quite right. In particular, Farrar believes that speed recorded was of the shock front relative to the surrounding gas, which is falling into the cluster. This means the data produced an estimate of the overall speed of the collision too high by some 1500 km/s (arXiv.org/astro-ph/0703232).

Farrar told Physics Web that it is too soon to know whether this will rule out the existence of a fifth force, as she will now have to reanalyse the Bullet Cluster using the updated measurements. Nevertheless, Douglas Clowe, a cosmologist from Ohio University in the US who has spent much time searching for evidence of dark matter in the Bullet Cluster, says that the test for the fifth force laid out by Farrar and Rosen will still have importance in the future. "Whether a fifth force is [ultimately] measured or we put a limit on how strong it could be will help in our continuing search for what dark matter actually is – but it is unlikely to be the final piece that identifies it."