What’s 250 Million Light-Years Big, Almost Empty, and Full of Answers?
BY MATTHEW R. FRANCIS
The darkest hole in the universe lies near the constellation Boötes, the plowman of the northern sky. This region, called a void, is 250 million light-years across and is almost entirely bereft of matter. If you centered a region of the same size on the Milky Way, you’d find thousands of galaxies. The Boötes void has only 60. Looking in all directions from its center, the cosmos would appear dark and empty.
If galaxies are cities, then the edges of a void are the suburbs, and the center is the deepest wilderness. But an atlas isn’t complete if it includes only cities and roads; it also needs the empty spaces in between. Scientists are increasingly entering the deep, dark wilderness of the void to complete their atlas, and in doing so are learning about other, greater dark presences in our universe.
Dark matter, for example, makes up 80 percent of the matter in our universe, but neither emits nor absorbs light. Because its gravitational force gathers together atoms to make galaxies and clusters of galaxies, though, the stars, gas, and dust that constitute the visible stuff of the cosmos reflect the underlying dark matter distribution. Where galaxies are thickest, dark matter is densest. The reverse is also true: In a region empty (or nearly so) of galaxies, dark matter should be sparse.
“Voids are vast, shallow troughs in the dark matter [distribution],” explains Joseph Clampitt, a post-doctoral researcher at the University of Pennsylvania. “Galaxies sit at the center of narrow, towering peaks.” If you use height as a stand-in for density, “a typical galaxy has the same height and footprint as the Empire State Building [and] a medium-size void would be a 3-foot-deep hole three times the size of Manhattan.” Find the voids, fill out your dark matter map.
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Sillier me! 