If you figure out all the contents of the Universe -- normal matter, dark matter, dark energy, radiation, neutrinos, spatial curvature, etc. Image credit: Quantum Stories, retrieved via http://cuentos-cuanticos.com/ From Sili on dark energy: \"Why do we have to interpret the cosmological constant as an energy inherent in space itself? Dark energy doesn't have to be a cosmological constant, or an energy inherent to space itself. Energy is typically measured in Joules, while the energy density is Joules-per-square-meter. Thanks for a great week of comments, and I'll see you back here soon for more wonders of the Universe!