A group of Israeli scientists have cracked a riddle of the universe that has been vexing researchers for years. Dark matter the stuff that seems to comprise most of the matter in the universe is very mysterious because it appears to be absent from some galaxies. But it is there after all, according to Hebrew University physicists. Cosmology Prof. Avishai Dekel and colleagues at the HU's Racah Institute of Physics will publish their important findings in the September 29 issue of the prestigious journal Nature. Dark matter is so called because it can't be seen directly. But astronomers believe that it exists because the motions of stars seem to show that they are being pulled by something more than merely the visible stars and dust. It has been estimated that dark matter comprises over four-fifths of the mass in the universe. No one knows what it is made of but it doesn't seem to be like any of the known forms of matter. The Israeli team investigated the claim, made in 2003 by a team of astronomers writing in Science, that the slow speeds of outlying stars in some galaxies are inconsistent with the presence there of dark matter. The HU team has shown that these stars can still move slowly, even if the galaxies contain large amounts of dark matter. Current theory holds that the dark matter is present in some types of galaxies namely, spiral-shaped ones but not in elliptical-shaped ones. Because most of the visible mass in a galaxy is concentrated in the center, stars at great distances from the center would be expected to move more slowly than stars closer in. Instead, stars in the outskirts of spiral galaxies seem to be the same at all distances from the center. The reason for this, according to the dark matter theory, is the presence of an enormous halo of unseen dark matter in and around the galaxy, which exerts a gravitational influence on the stars. According to the Israeli theory, elliptical galaxies ought to contain just as much dark matter as any other galaxy. But the paths of the slow-moving stars that have been seen in elliptical galaxies don't seem to bear the imprint of dark matter's gravitational tug, which would speed them up. Dekel and his team suggest that a dearth of dark matter in elliptical galaxies which are created when smaller galaxies merge would have been "especially puzzling in the context of the common theory of galaxy formation, which assumes that ellipticals originate from mergers of disk [spiral] galaxies." "Massive dark-matter halos are clearly detected in disk galaxies, so where did [the halos] disappear to during the mergers?" he asked. Dekel and scientists on his team performed computer simulations of the mergers in which elliptical galaxies were formed and found that these processes could produce stars in elongated orbits that move slowly even when the dark matter is present. The simulations were run on a supercomputer by graduate student Thomas Cox and supervised by physics Professor Joel Primack of University of California at Santa Cruz. The simulations were analyzed by Dekel and collaborators Felix Stoehr and Gary Mamon at the Institute of Astrophysics in Paris, where Dekel is the incumbent of the Blaise Pascal International Chair of Research at the Ecole Normale Superieure. The simulations show that the observations reported in Science were a predictable consequence of the violent collision and merger of the spiral galaxies that led to the formation of elliptical galaxies. "If we see a star at a large distance from the center of the galaxy, that star is going to be mostly moving either away from the center or back toward the center. Almost certainly, most of its motion is perpendicular to our line of sight," Dekel said. Under such circumstances, the star would appear to be moving quite slowly, when in fact this is not the case, based upon the models of simulated galaxy mergers studied by the Hebrew University-UCSC-Paris team. "Our conclusion is that what the cosmologists described in 2003 is exactly what the dark matter model would predict," he said, "Our findings remove a problem which bothered them and make it possible to better understand the processes involved in creation of new galaxies in the universe."