Observations of distant supernovae reveal that we live in an accelerating universe

• Observations of galaxy clusters suggest that the average density of matter in the universe is about 0.27 of the critical density

• The remaining contribution to the average density is called dark energy

• Measurements of Type Ia supernovae in distant galaxies show that the expansion of the universe is speeding up

• This may be due to the presence of dark energy in the form of a cosmological constant, which provides a pressure that pushes the universe outward