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.

The gravitational binding means that the galaxies and other material within a mature cluster have settled into an overall dynamic equilibrium. Galaxies buzz around within it and are kept from flying apart by dark matter, a mysterious form of matter that has eluded detection except through its gravitational effects. The interactions of these components produce a rich array of phenomena that astronomers are only beginning to grasp.

Like metropolises on Earth, clusters are more than the sum of their inhabitants. Processes occurring at the scale of a cluster can dictate events on much smaller scales, such as the growth of galaxies and the fueling of the supermassive black holes at the hubs of those galaxies. In turn, the black holes blow out huge amounts of high-speed material that can drive the evolution of the entire cluster. At first glance, these interconnections between large and small are enigmatic. The diameters of the black holes in question are smaller than the solar system. For them to affect an entire galaxy cluster would be like a blueberry affecting the entire Earth.

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