About 5 billion years ago, everything changed. The expansion of the universe, which had been gradually decelerating for billions of years, reversed course and entered into a period of unbridled acceleration. (It was sort of like a car that switches from decelerating to accelerating, but is still moving forward the whole time.) The unhurried, deliberate process of structure formation — the gradual buildup of ever-larger assemblies of matter from galaxies to groups to clusters — froze and began to undo itself.
Five billion years ago, a mysterious force overtook the universe. Hidden in the shadows, it lay dormant, buried underneath fields of matter and radiation. But once it uncovered itself, it worked quickly, bending the entire cosmos to its will.
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