![](http://www.symmetrymagazine.org/sites/default/files/styles/lead_image/public/images/standard/sky_survey-s.jpg)
About 5 billion years ago the universe underwent a crucial transition. The gravitational tug that pulled together the matter in the universe was overwhelmed by a different, repulsive phenomenon. As a result, the universe began to expand at an accelerating rate.
Scientists have given that phenomenon a name: dark energy. However, they can say with confidence only what it does, not what it is, where it comes from, or why it’s pushing galaxies apart at an ever more rapid speed.