The standard model of gravitation, GR, and its cosmological model, LCDM, require the existence of Dark Energy to fit cosmological observations.
In the 1970's it was realized that there were three problems with GR, the horizon problem, the density problem and the smoothness problem. Guth discovered the theory of Inflation by studying the effect of the Higgs scalar field on the early universe. The universe is believed to have exponentially expanded at an enormous rate to 10^60 times its previous size at about 10^-33 second after the Big Bang. This sudden inflation solved the three problems but required the universe to have a density equal, pretty well, to the critical density at which the space-time of the universe is flat.
However it seemed that not enough mass was around, only about 2-4% of that required. Dark Matter was invented, some sort of exotic particle that has not been discovered and this also helped to explain why galaxies and their clusters were revolving at too high a rate for the matter that could be seen. However even with DM not enough mass was around, only about 30% that required.
Next distant Type Ia supernovae were discovered to be fainter than theory said they should be. To explain this it seems as though the universe has accelerated in its expansion against the normal expectation that it slows down because of the gravitational attraction of one galaxy on another.
Finally the WMAP satellite looking at the fluctuations in the microwave background discovered the universe was flat after all. These were the major observations and there are several other minor ones. To make up the difference between matter, dark matter and the density needed to make the universe flat dark energy was invented, and perhaps it can explain why the universe is accelerating as well!
If you don't know what it is, don't worry, nobody does! We don't know what dark matter is either and Inflation requires a 'Higgs boson' fundamental particle that hasn't been discovered. So make of that what you will. Some say there are other explanations for these observations but the cosmological community is very sure that they all do exist and they are known with "precision". (4% matter, 23% dark matter and 73% dark energy)
So the LCDM model is robustly established, or so we think, and yet we have not the slightest idea what 96% of the universe is. That is the state of cosmology at the beginning of the 21st century!
Garth
