In 1933 the astronomer Fritz Zwicky unearthed a puzzle that has kept researchers busy ever since. He showed that galaxies in the Coma Cluster, the massive cluster of galaxies closest to us, are moving so fast relative to one another that they should have escaped – but yet there they are.

There are two possible solutions to this puzzle: either the stars that we observe are merely a tracer (about 1%) of the total amount of matter in the cluster and the rest is in the form of some kind of exotic “dark” matter; or gravity is much stronger on million light-year scales than the expected Newtonian 1/r2 force law. However, neither Zwicky’s observations nor those made in the intervening years have allowed researchers to distinguish conclusively between these solutions.

One of the difficulties in distinguishing between these two possibilities is that every possible “source” of gravity in a galaxy cluster occupies the same space. If we could take a cluster of galaxies into a laboratory here on Earth, then the obvious way of solving this problem would be to physically separate the various types of matter, weigh them individually and so determine which of the components is causing the required gravitational pull.

Fortunately, nature briefly does the equivalent of this in the early stages of a cataclysmic cosmic event called a cluster merger, during which two galaxy clusters gravitate towards one another until they collide and eventually produce an even bigger cluster. The forces involved in such an event are so great that the X-ray emitting plasma is “ripped” away from the galaxies and any dark matter that might be present within them.

With such a physical separation between the different matter components, we are then in a position to determine which component is exerting most of the gravity in the system and therefore whether an additional “dark” matter component is indeed present.

In the February issue of Physics World, Douglas Clowe and Dennis Zaritsky discuss how studies announced in August last year of one of these cluster mergers, nicknamed the "Bullet Cluster", have given astrophysicists some of the best evidence yet for the existence of dark matter.

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