The galaxy cluster 1E0657-56, also called the "Bullet cluster", is actually a group of two galaxy clusters that collided 150 million years ago. The "Bullet cluster" became famous in 2006. Researchers like Maxim Markevitch announced at the time that it was extremely convincing proof of the existence of dark matter, but on the other hand strong evidence against the MOND (Modified Newtonian Dynamics) theory invoked as an alternative explanation for dark matter for the rotation curves of galaxies and the stability of galaxy clusters.
False colour image of the "bullet cluster". In blue, the dark matter associated with the galaxy clusters, in red the hot gas emitting X-rays (Credit: NASA).