Dark matter and dark energy in the universe

For the first time, we have a plausible and complete accounting of matter a nd energy in the Universe. Expressed a fraction of the critical density it goes like this: neutrinos, between 0.3% and 15%; stars, between 0.3% and 0. 6%; baryons (total) 5% +/- 0.5%; matter (total), 40% +/- 10%; smooth, dark energy, 80% +/- 20%; totaling to the critical density (within the errors). This accounting is consistent with the inflationary prediction of a flat Un iverse and measurements of the anisotropy of the CBR. It also defines three "dark problems": Where are the dark baryons? What is the nonbaryonic dark matter? What is the nature of the dark energy? The leading candidate for th e (optically) dark baryons is diffuse hot gas; the leading candidates for t he nonbaryonic dark matter are slowly moving elementary particles left over from the earliest moments (cold dark matter), such as axions or neutralino s; the leading candidates for the dark energy involve fundamental physics a nd include a cosmological constant (vacuum energy), a rolling scalar field (quintessence), and a network of light, frustrated topological defects.