Gravity tested across cosmic distances, strengthening the case for dark matter

Astronomers have discovered that stars in the outer portions of galaxies tend to move much faster than expectedGravity is best known as the force that keeps us on Earth, but its effects extend throughout the cosmos. It influences the motions of stars, galaxies and clusters of galaxies, and helps determine the development and growth of the biggest structures in the universe. Yet astronomers have faced a puzzling problem for a long time. Stars and galaxies can sometimes seem to move far faster than their visible matter should allow. That discrepancy has spawned two competing ideas. Either the universe is filled with vast quantities of unseen dark matter, or the laws of gravity change on huge cosmic scales. Now one of the biggest tests ever has been carried out by University of Pennsylvania r