In the 1930s, astronomers discovered that many galaxy clusters observable from Earth have a much stronger gravitational field than they should have given their predicted mass. Further astronomical observations only added to this puzzle. After much consideration, it was concluded that something mysterious called dark matter must be involved. Dark matter is in all respects invisible and can only be detected by its gravitational effect on normal matter. If this new theory was right then dark matter would make up most of the mass of the universe.

However, in February three scientists claimed that dark matter was not necessary and in fact by slightly altering Einstein's equations for general relativity they could account for the acceleration. Not everyone was convinced by the new explanation though, and now new evidence has been put forward in support of dark matter through studying the "bullet" galaxy cluster with the Chandra X-ray telescope.

The cluster was created when two separate clusters smashed together. Tremendous amounts of energy were released in this collision; enough in fact to tear the normal matter and the dark matter apart. Even though dark matter is invisible, scientists were able to see the effect by measuring how the mass of the cluster was distributed.

The data gathered supported a model involving dark matter but not an altered form of general relativity as was previously proposed. No doubt the argument over the existence of dark matter will continue but supporters of the dark matter model believe this provides the most conclusive evidence yet.

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