Question: is the universe really full of dark energy? if so what causes it?

Arttu Rajantie answered on 16 Jun 2011:

Yes, it seems that roughly three quarters of the total energy are some kind of dark energy, which is invisible, very uniform and has negative pressure. It makes the expansion of the universe accelerate, and this was first observed by looking at very distant supernovae but there are now many different observations that confirm it.

The simplest explanation is that vacuum simply has non-zero energy density. As James points out, this is something that quantum field theory actually predicts, but then we would expect it to be massively higher, as James explains. The theory does not actually make a definite prediction, so smaller values are still consistent with it, but the smallness of the observed dark energy density does look very unnatural.

Even if we accept this explanation, we have a puzzle, because it is surprising that the density of dark energy is almost the same as the density of matter in the universe. (It is actually three times bigger, but that is small compared with the numbers we typically see in these calculations, such as 10^120.) The ratio of the matter and dark energy densities decreases as the universe expands, so this means that we appear to live at the special time in the history of the universe when the two are comparable. This is known as the coincidence problem.

The coincidence problem and the smallness of the dark energy density have prompted people to propose alternative explanations for dark energy in which its density is not constant but changes with time. I don’t think any of these explanations is particularly attractive, so at the moment my best guess would be that it is just vacuum energy, but we really do not know for sure.