Scientists believe that dark energy--the mysterious force that is accelerating cosmic expansion--makes up about 70 percent of the mass and energy of the universe. But because they don't know what it is, they cannot observe it directly.
Scientists believe that dark energy--the mysterious force that is accelerating cosmic expansion--makes up about 70 percent of the mass and energy of the universe. But because they don't know what it is, they cannot observe it directly.
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