 The Hubble Ultra Deep Field, a famous image of some of the most distant observed galaxies taken by the Hubble Space Telescope. (NASA) | | | Here's a spooky pre-Halloween thought: We have no idea what most of the universe is made of. Everything on Earth, every body in the solar system, every dust cloud and exoplanet and distant star humans have ever observed, constitutes just 5 percent of the mass and energy in the universe. Another 27 percent of what's out there is dark matter, the mysterious material that holds galaxies together. The remaining 68 percent is dark energy — a force that creates no light or heat, that no instrument can detect and no telescope can see. Yet scientists have direct evidence of dark energy's effect on the universe. When researchers make maps of the cosmos, they find that everything is moving away from everything else, and that the most distant galaxies are moving fastest. The universe is expanding at an accelerating rate, and dark energy — whatever it may be — is responsible. This month, scientists achieved "first light" for a new tool that will investigate the dark matter mystery. The Dark Energy Spectroscopy Instrument (nicknamed DESI) is designed to capture the light from more than 30 million -galaxies over a 11-billion-light year expanse of space. By separating the light into its component wavelengths, the instrument can determine a quality known as "red shift" — how much the light has been stretched out as the galaxy flies farther and farther away. This in turn can tell scientists about the clumping of matter in the early universe, and the force that has been pushing at it ever since. So far, it seems like the effects of dark energy are distributed equally across the cosmos, which would suggest it's some inherent quality of space itself — what Albert Einstein called "cosmological constant." But if DESI finds variations in dark energy over space and time, scientists may need to seek out more exotic explanations, like a new kind of fluid or field. Then there's the possibility that Einstein's theories of gravity — which have guided our understanding of the universe for a century — have a fundamental flaw. That would have implications not just for big picture cosmology questions, but for the behavior of normal matter in regions much closer to home. In probing the universe's darkest feature, we may uncover mysteries in parts of the cosmos we thought we understood. Spooky indeed. | 
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