Photo: At Ten, Dark Energy "Most Profound Problem" in Physics

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The spiral galaxy NGC 4258, 25 million light-years away, is seen in a composite image released by NASA in 2007. The galaxy is one of many that contain visible Type Ia supernovae, which astronomers use to tell how fast the universe is expanding.

Ten years ago in May astronomers announced that the universe's expansion is speeding up thanks to dark energy, a mysterious "antigravity" that makes up 74 percent of the universe.

X-ray image courtesy NASA/CXC/Univ. of Maryland/A.S. Wilson et al.; Optical image courtesy Pal.Obs. DSS; IR image courtesy NASA/JPL-Caltech; VLA radio image courtesy NRAO/AUI/NSF

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