Where is the rest of the universe ?

Scientists trying to create a detailed inventory of all the matter and energy in the cosmos run into a curious problem — the vast majority of it is missing. "I call it the dark side of the universe," said Michael Turner, a cosmologist at the University of Chicago, referring to the great mysteries of dark matter and dark energy. In fact, only 4 percent of the matter and energy in the universe has been found. The other 96 percent remains elusive, but scientists are looking in the farthest reaches of space and deepest depths of Earth to solve the two dark riddles. Einstein's famous equation "E=mc^2" describes energy and matter (or mass) as one and the same — maps of the cosmos refer to the energy-matter combination as energy density, for short. The problem with detecting dark matter, thought to make up 22 percent of the universe's mass/energy pie, is that light doesn't interact with it.

But it does exhibit the tug of gravity. Initial evidence for the mysterious matter was discovered 75 years ago when astrophysicists noticed an anomaly in a jumble of galaxies: The galactic cluster had hundreds of times more gravitational pull than it should have, far outweighing its visible mass of stars. "We can predict the motions of the sun and planets very accurately, but when we measure distant things we see anomalies," said Scott Dodelson, an astrophysicist at Fermi National Accelerator Laboratory in Illinois. "Dark matter is currently the best possible solution, even though we've never seen any of it."

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