  This chapter aims to present an introduction to current research on the
nature of the cosmological dark matter and the origin of galaxies and large
scale structure within the standard theoretical framework: gravitational
collapse of fluctuations as the origin of structure in the expanding universe.
General relativistic cosmology is summarized, and the data on the basic
cosmological parameters ($t_o$ and $H_0 \equiv 100 h \kmsmpc$, $\Omega_0$,
$\Omega_\Lambda$ and $\Omega_b$) are reviewed. Various particle physics
candidates for hot, warm, and cold dark matter are briefly reviewed, together
with current constraints and experiments that could detect or eliminate them.
Also included is a very brief summary of the theory of cosmic defects, and a
somewhat more extended exposition of the idea of cosmological inflation with a
summary of some current models of inflation. The remainder is a discussion of
observational constraints on cosmological model building, emphasizing models in
which most of the dark matter is cold and the primordial fluctuations are the
sort predicted by inflation. It is argued that the simplest models that have a
hope of working are Cold Dark Matter with a cosmological constant
($\Lambda$CDM) if the Hubble parameter is high ($h \gsim 0.7$), and Cold + Hot
Dark Matter (CHDM) if the Hubble parameter and age permit an $\Omega=1$
cosmology, as seems plausible in light of the data from the Hipparcos
astrometric satellite. The most attractive variants of these models and the
critical tests for each are discussed.
