via DavidReneke:
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....
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.
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.
Five billion years ago, dark energy awoke. [The Universe: Big Bang to Now in 10 Easy Steps]
he then goes on to explain what this is and what it means: it keeps all the galaxies from clumping together in a big mush:
And once dark energy took over, the game changed completely. Because of the constant nature of its density, compared to the lowering density of matter, expansion not only continued but also accelerated. And that accelerated expansion halted the process of structure formation: Galaxies would love to continue gluing onto each other to form larger structures like clusters and superclusters, but the intervening empty space is inexorably pulling them apart.
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