[Link] It seems that the Big Bang expansion was not uniform. At one point gravity was overwhelmed and the expansion accelerated.
"So after the big bang, matter was still relatively dense in the Universe and therefore gravity braked expansion. But as galaxies moved farther apart, dark energy began to exert a more significant influence. For a brief period, two forces balanced and 'the expansion of the Universe coasted along at a steady rate, like a car in cruise control,' says Riess.
But then, five billion years ago, dark energy got the upper hand. No-one yet knows what dark energy is, but supernova observations provides some constraints. 'It gives a handle on how much there is,' says Saul Perlmutter of Lawrence Berkeley National Laboratory in California."
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