Dark Matter & Dark Energy
A number of independent probes of the geometry and expansion history of the Universe agree that a single cosmological model can fit all observations, with only 4.9 percent of the energy content of the Universe being accounted for by ordinary matter. 26.8 percent of the energy content appears to be in the form of "dark matter", which only interacts with ordinary matter and radiation through gravity. 68.3 percent appears to be in a form of "dark energy", which causes the expansion of the Universe to accelerate. The main aims of modern observational cosmology are to test whether this model is correct, to search for discrepancies, and to obtain higher precision measurements of the parameters of the cosmological model, so that theories of how the Universe came to have this composition may be tested.
The observational cornerstones of this model of the Universe are the cosmic microwave background fluctuations, the large-scale structure of the galaxy distribution, the Hubble diagram obtained from Type IA supernovae, and measurements of gravitating matter using gravitational lensing.