The beautiful spinning pinwheel of the Andromeda galaxy, our celestial neighbor, poses a mystery. The breakneck speed of its rotation cannot be explained by applying the known laws of physics to the disk's visible matter. By rights, the gravity generated by the galaxy's apparent mass should cause the stars in the periphery to move more slowly than they actually do. If the visible matter was all there was, Andromeda, and nearly all such quickly rotating galaxies, simply should not exist.
Cosmologists believe that some unseen kind of matter—dark matter—surrounds and permeates Andromeda and other galaxies, adding the necessary gravitational force to keep them spinning as observed. Dark matter, which appears to contribute about 25 percent of the universe's mass, would also explain other aspects of the cosmos, including the exceedingly fast motion of galaxies within clusters of galaxies, the distribution of matter arising when two clusters collide and the observation of gravitational lensing—the bending of light by gravity—of distant galaxies.
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