http://www.skyandtelescope.com/news/36372969.html
"Throw a baseball up toward the sky, and gravity will slow its travel from the moment it leaves your hand. For decades, astronomers assumed that the post-Big Bang universe worked the same way. Even though galaxies are flying apart as space expands, their motion should be decelerating as the eons go by, due to the pull of their gravity on each other.
But a decade ago, cosmologists discovered something totally unexpected. The expansion of the universe has not been slowing down but
speeding up in the past few billion years. It's as if the baseball you threw upward, instead of slowing down, suddenly sprouted a rocket engine and took off toward the clouds.
Something akin to anti-gravity — dubbed "dark energy" for lack of a better term — has apparently been inflating space. The evidence? Extremely far-off galaxies are traveling away from us at the wrong speeds (as measured by their redshifts) for their distances (as measured by the brightnesses of supernovae within them)..."