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Somewhere far out in the universe, something is exerting a tremendous pull on the Milky Way galaxy, which includes the solar system, and most of its neighboring galaxies, astronomers have found. The unexpected discovery may force a revision of some basic notions about the universe.

What is tugging at these galaxies is not known, but it may be invisible matter clumped on much larger scales than can be readily explained by any current theory.The astronomers who discovered the magnitude of these peculiar galactic motions say their observations "strongly challenge our understanding of how the universe evolved."

The first reactions of scientists to the findings have run from astonishment to skepticism to earnest debate over the implications for theories straining to answer one of the most puzzling questions in cosmology: how did the universe evolve from an early state of virtual homogeneity to the observed lumpy conditions today in which stars congregate in galaxies, galaxies in clusters and clusters of galaxies in superclusters that stretch across hundreds of millions of light-years of space?

These theories are based on the well-established idea that some unknown invisible matter, called dark matter, accounts for more than 90 percent of the mass of the universe and presumably helps account for its structure.

But many of these theories were already tottering and undergoing drastic revisions. Since the new findings indicate that the universe has clumps of matter, or structure, on much larger scales than had been predicted, cosmologists may be facing intellectually turbulent times searching for more satisfying theories.

The unsettling discovery was made by Tod R. Lauer of the National Optical Astronomy Observatories in Tucson, Ariz., and Marc Postman of the Space Telescope Science Institute in Baltimore.

They conducted a study of galaxy motions over the entire sky out to distances of more than 500 million light-years. The results, which have been widely discussed in seminars, will be published in the April 20 issue of The Astrophysical Journal.

The two astronomers found that in addition to moving with the general expansion of the universe , the Milky Way and the nearby universe appear to be drifting in a particular direction with respect to the more distant universe.