John Baez, Whats the Energy Density of the Vacuum?
• Sean Carroll, The Cosmological Constant. The third is the most detailed, and it has lots of good references for further study. • Why does it seem like the gravitational mass of galaxies exceeds the mass of all the stuff we can see, even taking into account our best bets about invisible stuff like brown dwarfs, “Jupiters”, and so on? Is there some missing “dark matter”? If so, is it ordinary matter, neutrinos, or something more exotic? If not, is there some problem with our understanding of gravity, or what? Since the late 1990s, a consensus has emerged that some sort of “cold dark matter” is needed to explain all sorts of things we see. For example, in 2003 an analysis of data from the Wilkinson Microwave Anisotropy Probe suggested that the energy density of the universe consists of about 23% cold dark matter, as compared to only 4% ordinary matter. (The rest is dark energy.) Unfortunately nobody knows what this cold dark matter is! It probably can’t be ordinary matter we’ve neglecte
