Dark energy is the name used by scientists to describe whatever it is that is pushing the universe apart. Since the 1920’s, astronomers have been able to see that the universe is expanding, and this knowledge led them to the Big Bang Theory. One of the Hubble Space Telescope’s most amazing discoveries was that this expansion is not slowing down (as you would expect, because of gravitational attraction), but actually speeding up. Nothing that we know about physics can explain this, although modern physicists are full of ideas that need testing. Until they figure it out and are able to come up with a suitably serious scientific name, we call it Dark Energy.