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Astronomers think that the expansion of the universe is regulated by both the force of gravity, which acts to slow it down, and a mysterious dark energy, which pushes matter and space apart at faster and faster speeds. In this artist's conception, dark en (Image: NASA/JPL-Caltech) Astronomers think that the expansion of the universe is regulated by both the force of gravity, which acts to slow it down, and a mysterious dark energy, which pushes matter and space apart at faster and faster speeds. In this artist's conception, dark energy is represented by the purple grid above and gravity by the green grid below. Gravity emanates from all matter in the universe, even dark matter.