It turns out that roughly 68% of the universe is dark energy. Other than that, it is a complete mystery. We know how much dark energy there is because we know how it affects the universe's expansion. Theorists still don't know what the correct explanation is, but they have given the solution a name. Maybe there is something wrong with Einstein's theory of gravity and a new theory could include some kind of field that creates this cosmic acceleration.

Maybe it was a result of a long-discarded version of Einstein's theory of gravity, one that contained what was called a 'cosmological constant.' Maybe there was some strange kind of energy-fluid that filled space. But something was causing it.Įventually theorists came up with three sorts of explanations. No one expected this, no one knew how to explain it. So the expansion of the universe has not been slowing due to gravity, as everyone thought, it has been accelerating. Then came 1998 and the Hubble Space Telescope (HST) observations of very distant supernovae that showed that, a long time ago, the universe was actually expanding more slowly than it is today. The universe is full of matter and the attractive force of gravity pulls all matter together. Granted, the slowing had not been observed, but, theoretically, the universe had to slow. It might have enough energy density to stop its expansion and recollapse, it might have so little energy density that it would never stop expanding, but gravity was certain to slow the expansion as time went on. In the early 1990s, one thing was fairly certain about the expansion of the universe.

The things we say in the dark