Walk outside on any clear evening and look at the sky. Unless you live in a light-polluted metropolis, you’ll be dazzled by all the bright points of light dotting the sky. With a telescope, you can spot glowing clouds of gas, clusters of stars, even the faintly glowing disks of distant galaxies. It’s easy to believe that the objects giving off the light dominate our universe, but the reverse is actually true. Cosmologists have found that dark matter—stuff that gives off little or no light—makes up the lion’s share of the heavens. Even more importantly, cosmologists realize that the amount of dark matter will ultimately determine the fate of the universe.

       What governs the final destiny of the cosmos? All the matter in the universe exerts a gravitational tug that tries to pull the galaxies back together again. But the universe is expanding rapidly, so it will take an awful lot of mass to halt the expansion. It’s like trying to launch a spacecraft from the surface of Earth.

       In the same way, the total mass of the universe determines whether the outrushing galaxies will continue on forever or one day slow down, stop, and then reverse directions. If there's enough mass to cause the galaxies to stop and then begin contracting in the far future, cosmologists say the universe is closed. If the mass is too small to halt the expansion, the universe is said to be open. In the intermediate case where the universe continues expanding but at a speed that ultimately approaches zero, the universe is flat. (Many cosmologists believe that the universe underwent a period of very rapid expansion, or inflation, early in its history. If true, the universe should be flat.)