... No one was quite sure, but the answer is giving clues to the nature of mysterious dark matter . In the case of MACSJ0025.4-1222 , two huge clusters of galaxies have been found slowly colliding over hundreds of millions of years, and the result has been imaged by both the Hubble Space Telescope in visible light and the Chandra Space Telescope in X-ray light . ... Publications with keywords: cluster of galaxies - dark matter . Publications with words: cluster of galaxies - dark matter . ...