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Many of the universe’s intergalactic clouds of bright, X-ray-emitting matter are composed of lightweight electrons rather than heavier gases, which means that the universe is about 20 percent less massive than previously thought.

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Many of the universe’s intergalactic clouds of bright, X-ray-emitting matter are composed of lightweight electrons rather than heavier gases, which means that the universe is about 20 percent less massive than previously thought.

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