Situated over 2,000,000,000 two billion)light-years from Earth, galaxies in cluster Abell 2390 (top) andMS2137.3-2353 (bottom are seen in the right hand panels above,false-color images from th HubbleSpace Telescope.Corresponding panels on the left reveal each cluster' x-rayappearance in images from the Chandra X-ray Observatory.While the Hubble images record the cluster's star-filled galaxies,the x-ray images show no galaxies at all ... onlymulti-million degree hot intracluster ga which glows in high energy x-rays.But there lies a profound mystery.The total mass in the galaxies on the right, plus themass of the hot gas on the left, falls far short of providing enough gravityto confine the hot gas withi the galaxy clusters.In fact the best accounting to datecan only find 13 per cent (!)of the total matter necessary Gravitational lensarcs visible in the deep Hubble imagesalso indicate these clusters have much more mass than directly identifiablein the Chandra and Hubble data.Astronomers conclude that most of the cluster matter i dark matter,invisible even to the combined far-seeing eyes of these orbitin astrophysical observatories.What is thenature of this cosmic dark matter?
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