Scientists think that a halo of dark matter surrounds the Milky Way, extending out far beyond its visible edge, as occurs at other galaxies. .In the new study, researchers used data from Gaia, a European Space Agency mission mapping the positions of billions of stars, to study the Hercules Stream, a thick cluster of stars that revolve around the Milky Way at the same rate that the galactic bar itself spins. .Because the stars in the Hercules Stream are gravitationally trapped by the pivoting bar, slowing down the bar's rotation would cause the stars to creep outward to keep their orbits in sync with the bar's spin.Astronomers believe that dark matter shrouds the Milky Way — and other galaxies — in an elusive halo that extends far out into space."Our research provides a new type of measurement of dark matter — not of its gravitational energy but of its inertial mass (the dynamical response), which slows the bar's spin," Schoenrich said.
"Our finding also poses a major problem for alternative gravity theories — as they lack dark matter in the halo, they predict no, or significantly too little slowing of the bar.".
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