Physicists believe that around 70% of the Universe is what they call dark energy. What do they mean by that and how do they know? To find out, here's an extract from a Plus article by the cosmologist and mathematician John D. Barrow.
"During the 1990s the Hubble Space Telescope and new detector developments in astronomy allowed researchers to see further than ever before in the Universe. They were able to see supernovae [exploding stars] almost at the edge of the visible Universe. By photographing a region of the Universe twice at an interval of a week using telescopes on Earth, astronomers discovered that within this period many stars had exploded in supernovae. They then used the Hubble Space Telescope and ground-based telescopes to see how the light from the exploded stars varied as [the supernovae] reached their maximum brightness and then faded away.
This movie shows the light curve and spectrum of a supernova (marked by the red arrow) that was observed in the Centaurus A galaxy. Movie courtesy the Supernova Cosmology Project.
What was very unexpected about these observations was that the characteristic signature — the light curve as it's called — of the brightening and dimming of these supernovae was exactly the same as the signature of a big class of supernovae which we could see locally and understand quite well. This told us that these very distant supernovae are intrinsically the same as nearby ones. The only difference in their appearance is that they look dimmer, simply because they are far away.
This gave astronomers an exciting new way of determining the distance to far away objects: if you understand a supernova's workings, you know its intrinsic brightness and so if you measure its apparent brightness you can deduce how far away it is. You can therefore extend what's known as Hubble's law to a far greater distance than ever before. Hubble's law describes the fact that, in our expanding Universe, the speed with which galaxies and stars are receding away from the Earth is proportional to their distance from us.
This led to an extremely unexpected and revolutionary discovery. As you go out far enough, the expansion of the Universe starts to speed up. Different research groups scrutinised this result in great detail and discovered that the Universe's expansion was indeed accelerating. And to explain that acceleration, around 70% of the Universe has to be in a form of mass and energy that's able to create the acceleration.
The 70% of mysterious stuff in the Universe that [drives the accelerated expansion] is called dark energy. We don't know what it is, in the sense that we can't identify it yet as being a particular type of matter that is predicted by some part of particle physics. But there are many possibilities for what it could be, perhaps too many."
To find out what these possibilities are, and what other evidence we have for dark energy, read Barrow's full article.