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191 absence of stars in dark nebulae might be real; only in the twentieth century did they infer that invisible matter was diminishing the light of many stars.‘ A generation ago, astronomical textbooks stated that the Milky Way contains about 100 billion stars; newer estimates are as high as 400 billion. New astronomical capabilities continue to reveal structures that had been hidden from us. Galaxies that are detectable only in submillimeter wavelengths were not discovered until 1997, even though they are some of the brightest objects in the universe.* Do we now stand on the threshold of knowing everything? Or will our latest models of the cosmos be rejected by our descendants? Twentieth-century science revealed that the universe is much stranger than we had imagined. Scientists now believe that visible matter constitutes less than 1% of the universe’s matter and energy. If we add together all forms of familiar (baryonic) matter, including gas in galaxy clusters and the intergalactic medium, we still come up with less than 5%.° Roughly a quarter of the universe is composed of dark (nonbaryonic) matter, which we can detect only by its gravitational effects. The Massive Compact Halo Object survey concluded that between 8% and 50% of dark matter in our Galaxy is in clumps weighing about half the mass of the sun (Some astronomers believe that these are the burned-out normal stars known as white dwarfs). Black holes are thought to make up only a small part of the total.’ As of early 2006, the most popular theory still proposed that most dark matter consists of massive exotic particles that do not interact with normal matter except through gravity. If such dark matter is truly different from ordinary matter, we are made of atypical material. As Trefil put it, the kind of matter that makes up our solar system, our Earth, and our bodies is a relatively minor part of a universe that is composed predominantly of very different stuff.* In a sense, this could take us back to the pre-Copernican view that the heavens were made of different substances than our familiar world. Others speculate that dark matter is not actually matter at all; it might manifest a change in Newton’s laws at large distances. Astronomers and physicists are designing detectors that might find more direct evidence of dark matter. If they succeed, we may suddenly discover a quarter of the universe that had been hidden from us—one of the great achievements of twenty-first-century science. Yet most of reality still may be beyond our reach. Since 1998, “dark energy” has been seen as one of the central features of the cosmos, comprising more than 70% of the universe’s total matter and energy.'” The effect of this mysterious force is profound. For most of the past century, astronomers had assumed that gravity would slow down the expansion of our universe, perhaps contracting it back to a Big Crunch. That paradigm was reversed in the late 1990s when astronomers discovered that the expansion of the universe is speeding up, Misunderstanding the Universe