Until now the existence of dark matter was inferred by the fact that galaxies have only one-fifth of the visible matter needed to create the gravity that keeps them intact. So the rest must be invisible to telescopes. In a word, that unseen matter is "dark."
The observations of the Bullet Cluster, officially known as galaxy cluster 1E0657-56, do not explain what dark matter is. They do, however, provide one solid little hint, says Douglas Clowe, a researcher at the University of Arizona in Tucson.
"We can place some constraints on dark matter particles," said Clowe. It appears that dark matter particles, whatever they are, behave more like the raisins than the oatmeal -- they are either widely spaced, like stars, or have some other way of avoiding collisions with each other.
It's a small clue, says Clowe, but seeing it play out in the Bullet Cluster makes it an unusually solid clue for what's so far proven to be the most mysterious stuff in the universe.
"The great news about this is that it shows once and for all that dark matter exists," said physicist Sean Carroll of the University of Chicago. And that means, he said, there's less need to tweak Einstein's laws of gravitation to explain what's seen in galaxies.