  For the first time, we have a plausible and complete accounting of matter and
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 Universe
and defines three dark-matter problems: Where are the dark baryons? What is the
nonbaryonic dark matter? What is the nature of the dark energy? The leading
candidate for the (optically) dark baryons is diffuse hot gas; the leading
candidates for the nonbaryonic dark matter are slowly moving elementary
particles left over from the earliest moments (cold dark matter), such as
axions or neutralinos; the leading candidates for the dark energy involve
fundamental physics and include a cosmological constant (vacuum energy), a
rolling scalar field (quintessence), and a network of light, frustrated
topological defects.
