Between the distant quasars and the stars of the Sun's neighborhood lies a vast middle ground where the architecture of the cosmos stands in full display. Far from scattering at random as astronomers once surmised, galaxies tend to congregate, as people do. Moreover, their congregations also congregate. and so on. These vast clouds of organization--in which galaxies behave like swarming dust motes--have taught astronomers that there's much more to the universe than meets the eye. An astronomer can examine a galaxy and from its brightness make a guess as to how many stars and how much dusty gas it contains. Yet when we track the movements of galaxies in sef-contained cluster, they move as if each galaxy were far heavier than it appears. It's like watching go-carts move as unstoppably as trucks. What is this mass that looms invisibly among the bright lights of the galaxies? No one knows.
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