The present overall density of the Universe is very low, roughly 9.9 × 10 grams per cubic centimetre. This mass-energy appears to consist of 73% dark energy, 23% cold dark matter and 4% ordinary matter. The density of atoms is about a single hydrogen atom for every four cubic meters of volume. The properties of dark energy and dark matter are not known. Dark matter slows the expansion of the Universe. Dark energy makes its expansion faster.