Number 107 (Story #2), December 18, 1992 by Phillip F. Schewe and Ben Stein THE LARGE SCALE STREAMING OF GALAXIES presents some problems for cosmologists. In the 1970s and 80s scientists came to realize that the motion of galaxies could not all be attributed to the expansion of the universe. For example, a net flow of nearby galaxies has been attributed to the gravitational pull of a large concentration of matter---the Great Attractor---150 million light years away in the constellation Hydra. A new study of 120 galaxy clusters by Tod Lauer of Kitt Peak and Marc Postman of the Space Telescope Science Institute looks at this streaming on a larger scale. They find that in order to ensure that the brightest galaxies in these clusters all have the same intrinsic brightness (as independent studies suggest) then the velocity of our own galaxy relative to the cosmic microwave background (CMB) would be different from the velocity deduced using the east-west asymmetry (dipole moment) in the CMB measured by COBE. Lauer and Postman offer two tentative explanations of this velocity discrepancy: first, that the expansion of the universe is actually asymmetric and the dipole part of the measured CMB cannot be subtracted to "correct" for the motion of the Milky Way toward other galaxies; or, second, that the universe is symmetrical after all but that mass concentrations even larger (100,000 galactic masses) than the Great Attractor must loom at distances of 300 million light years. (Science News, 12 Dec.)