  For the first time, we have a plausible, 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, 0.5%; baryons (total), 5%;
matter (total), 40%; smooth, dark energy, 60%; adding up to the critical
density. 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 light, frustrated
topological defects.
