Does Dark Matter Really Exist?; August 2002; Scientific American Magazine; by Mordehai Milgrom; 8 Page(s)

OF ALL THE MANY MYSTERIES of modern astronomy, none is more vexing than the nature of dark matter. Most astronomers believe that large quantities of some unidentified material pervade the universe. Like a theater audience that watches the herky-jerky gestures of a marionette and infers the presence of a hidden puppeteer, researchers observe that visible matter moves in unaccountable ways and conclude that unseen matter must be pulling the strings. Yet this dark matter has eluded every effort by astronomers and physicists to bring it out of the shadows. A handful of us suspect that it might not really exist, and others are beginning to consider this possibility seriously.

The dark matter problem arose because of a mismatch in the masses of galaxies and larger cosmic structures. The constituents of these systems-stars and gas in the case of galaxies, gas and galaxies in the case of galaxy clusters-move about but do not escape, because they are checked by the gravitational pull from the rest of the system. The laws of physics tell us how much mass has to be present to counterbalance the motions and thereby prevent the dispersal of the system. Disconcertingly, the tally of mass that astronomers actually observe falls far short of that.