Some updates on the Bullet Cluster / dark matter saga. You can now see a picture of the Bullet Cluster here: http://math.ucr.edu/home/baez/week238.html I've also updated a bunch of information and fixed some mistakes in the initial version of "week238", thanks in part to Matt Owers. >Markevitch and company have been studying the "Bullet Cluster", a >a bunch of galaxies that has a small bullet-shaped subcluster zipping >away from the center at 4,500 kilometers per second. >It seems that one of the rapidly moving galaxies in this subcluster >has hit a bystander galaxy - I'm not sure, but a high-speed collision >of galaxies occurred. In fact, the whole subcluster hit another subcluster! So, the picture is a picture of colliding bunches of galaxies. The individual galaxies in these bunches mainly shoot right past each other - but the intergalactic gas in one bunch is hitting the gas in the other, and getting so hot it emits X-rays. >So, dark matter is seeming more and more real. In fact, last year >folks found evidence for "ghost galaxies" made mainly of dark matter >and cold hydrogen, with very few stars: > >6) PPARC, New evidence for a dark matter galaxy, >http://www.interactions.org/cms/?pid=1023641 Apparently the consensus is now that this ghost, VIRGOHI 21, is hydrogen stripped off from a galaxy by the "wind" it felt as it fell into the Virgo Cluster. This effect is called "ram pressure stripping" - the gas of a galaxy can be stripped off if the galaxy is moving rapidly through a cluster, due to interaction with the gas in the cluster.