Sorry if this sounds a bit mixed up. When I was growing up, in the late 1990's, popular science books about cosmology use to describe the average mass-energy density of the universe, especially comparing it to the critical density. Those books used to say that visible matter gives too-low density, but astrophysical evidence show that there is some other stuff out there that brings the density close to the critical value. IIRC, that's what called now "dark matter". Does this relate to dark energy at all? I see two conflicting points here: 1. Dark energy is said to be similar to the cosmological constant. The effect of that is similar to spreading a uniform negative mass over the universe. 2. Dark energy is said to account for 73% of the total mass-energy of the universe. I'm only a mathematician, and never learned GR formally, but these seem to conflict - is the energy of dark energy positive of negative? As an additional question, I would like to ask how the 73% estimate was computed. My guess is that the intensity of CMB radiation was measures, and the mass-energy density inferred from its temperature only gave 0.27Ω. Is this the way it's done?