Steve - you seem to have the same mental block as maddad- assuming dark matter is some exotic, unknown form of matter.<br /><br />Reputable Science Encyclopedias show otherwise. For example:<br /><br />Here is a quote concerning dark matter, which notes evidence for dark matter in Milky Way and also highlights known forms of matter:<br /><br />"Stars closer to the galactic center have shorter orbital periods than those which are farther away, but they do not behave like planets around the Sun. The mass of the galaxy is spread out over a vast volume and the speed of a star is determined not only by its distance, but also by the mass which lies between it and the center. The greater the distance of the star the greater the mass acting upon it.<br /> Close to the galactic center, velocities are quite high. They decrease towards the outer fringe of the nucleus, and then farther out they increase to about 230km per second at the Sun's distance, and to some 300km per second at a radius of 60,000 light years.<br /><br />The Galactic Corona<br />The high speeds of stars in the outermost parts of the galactic disk imply that a large fraction of the Galaxy's mass lies in the fringes of the system. If this were not so, the speeds of stars would begin to decrease at distances beyond that of the Sun. One RR Lyrae star, nine globular clusters, 3 dwarf elliptical galaxies, and the two Magellanic Clouds are known to lie at distances between 65,000 and 200,000 light years from the galactic center. If, as appears to be the case, they are all part of the halo, then it is possible to calculate from their speeds how massive the galaxy must be in order to prevent these objects from escaping into intergalactic space. This mass turns out to be between 1,000 billion and 2,000 billion solar masses.<br /> This mass consists of neither luminous stars nor neutral hydrogen, or it would be visible. Possibly it consists of cool planet-sized lumps of matter, old dead stars, or very low-mass st