In the past year, a radical shift has begun to occur. Until recently,
a majority of the theoretical community viewed the (standard)
Einstein-de Sitter model (_{0} = 1, _{} = 0) as
the most likely case (with *h* = 0.5, *t* = 13 Gyr). With
accumulating
evidence for a low matter density, difficulty in fitting the galaxy
power spectrum with the standard model, the conflict in ages for the
Einstein-de Sitter case, and now, most recently, the evidence from
type Ia supernovae for an accelerating universe, a new ``standard''
model is emerging, a model with _{m} ~ 0.3, _{}
~ 0.7, *h* = 0.65, and *t* = 13 Gyr. This model preserves a flat
universe and is still consistent with inflation.

In Figure 7, the bounds on several cosmological
parameters are
summarized in a plot of the matter density as a function of the Hubble
constant. What do these current limits on cosmological parameters
imply about the contribution of non-baryonic dark matter to the total
matter density? As can be seen from the figure, for *H*_{0} = 70
km/sec/Mpc, current limits to the deuterium abundance
(Burles & Tytler 1998;
Hogan 1998)
yield baryon densities in the range of
_{b} = 0.02 to 0.04, or
2-4% of the critical density. Given
current estimates of the matter density (_{m} ~ 0.3),
non-baryonic matter would thus contribute just over 25% of the total
energy density needed for a flat, = 1 universe.

One might ask, is non-baryonic dark matter still required if
is non-zero? Allowance for
0 does not provide the
missing energy to simultaneously yield = 1, while doing away
with the necessity of non-baryonic dark matter, at least for the
current limits from Big Bang nucleosynthesis. As can be seen from
Figure 7, for the current deuterium limits,
having all baryonic mass
plus would require both
*H*_{0} ~ 30 km/sec/Mpc and an
age for the Universe of ~ 30 Gyr. These *H*_{0} and age values are
outside the range of currently measured values for both of these
parameters. Although it might be appealing to do away simultaneously
with one type of unknown (non-baryonic dark matter) while introducing
another parameter (), a
non-zero value for the cosmological
parameter does not remove the requirement for non-baryonic dark
matter.

The question of the nature of dark matter (or energy) remains with us. In this sense, the situation has not changed very much over the past few decades, although the motivation for requiring a critical-density universe has evolved from considerations of fine-tuning and causal arguments to the development of inflation. But searches for dark matter since the 1970's have not uncovered sufficient matter to result in a critical-density universe. This year has offered exciting new (and therefore still tentative) results that a non-zero value of the cosmological constant, or perhaps an evolving scalar field like quintessence (Steinhardt, this volume; Steinhardt & Caldwell 1998) could provide the missing energy to result in a critical-density universe. Still, the nature of the dark matter, whether it contributes 25% or 95% of the total energy density, is unknown, and remains as one of the most fundamental unsolved problems in cosmology.

The progress in measuring cosmological parameters has been impressive; still, however, the accurate measurement of cosmological parameters remains a challenging task. It is therefore encouraging to note the wealth of new data that will appear over the next few years, covering very diverse areas of parameter space. For example, measurement of CMB anisotropies, (from balloons and space with MAP, and Planck), the Sloan Digital Sky Survey, Hubble Space Telescope, Chandra X-ray Observatory, radio interferometry, gravitational lensing studies, weakly interacting massive particle (WIMP) cryogenic detectors, neutrino experiments, the Large Hadron Collider (LHC), and a host of others, inspire optimism that the noose on cosmological parameters is indeed tightening. At the very least, the next few years should continue to be interesting!

It is a great pleasure to thank the organizers of this Nobel Symposium for a particularly enjoyable and stimulating conference, and for their immense hospitality. I also thank Brad Gibson for his help with the LMC distance literature search.