Culling clues from X rays emitted by distant clusters of galaxies, astronomers report new evidence that some mysterious force overcame gravity's tug about 6 billion years ago and ever since has been pushing galaxies apart at an accelerating rate. The results add to previous evidence for one of the strangest properties of the cosmos: Cosmic expansion is speeding up, rather than slowing down, in response to the mutual gravity of all material in the universe.
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