If you drew a large-scale map of the universe, it would look rather like a map of the U.S. Interstate Highway System. Galaxies line up in filaments that crisscross intergalactic space like freeways. In between the roads are regions of relatively low density: the cosmic countryside. And at the crossroads, where multiple filaments converge, are clusters of galaxies: the cosmic megacities.

The size of these clusters is daunting. It takes light a little more than a second to reach Earth from the moon and eight minutes to reach Earth from the sun. Light from the center of our Milky Way galaxy must make a journey of 25,000 years to reach us. Even that is fairly quick compared with the time required for light to cross a galaxy cluster--about 10 million years. In fact, clusters are the largest gravitationally bound bodies in the universe. The roadlike filaments may be larger in sheer size, but they are not coherent bodies held together by gravity.