They say that the total energy of the universe consists of 4.9\% ordinary matter, 26.8\% dark matter and 68.3\% dark energy. I hear this pie part in virtually every lecture in cosmology. How do we know these numbers?
These numbers are determined by fitting precise observational data of the small anisotropies of the cosmic microwave background, gathered by the Planck spacecraft, to the standard model of cosmology, the Lambda-CDM model.
In this successful model based on the Friedmann equations, there are three ingredients that make up the contents of our universe: dark energy (that’s the “Lambda”), “cold” (i.e., non-relativistic) dark matter (that’s the “CDM”), and ordinary matter. Radiation provides a negligible fraction of the current energy density.