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\def\lap{\lower.5ex\hbox{$\; \buildrel < \over \sim \;$}} \def\gap{\lower.5ex\hbox{$\; \buildrel > \over \sim \;$}}
Attempts to fill in some of the details of the standard hot expanding world picture by appeal to the observational evidence lead to some fascinating dilemmas, many related to the puzzles of the nature and amount of the dark matter. Well tested approaches yield two quite different values for the distance scale, $h\sim 0.5$ and $h\sim 0.8$ (where Hubble's constant is $H_o=100h$~km~s$^{-1}$~Mpc$^{-1}$). If the shorter scale proved to be right the expansion timescale in an Einstein-de~Sitter universe would be unacceptable; we would have to live in a low density universe, maybe with a significant cosmological constant. This is in the direction suggested by dynamical studies on scales $\lap 10h^{-1}$~Mpc, which quite consistently indicate a mean mass density $\Omega\sim 0.1$ times the critical Einstein-de~Sitter value. Dynamical measures on larger scales are less well explored, but there is a reasonable case that $\Omega$ is close to unity, as could fit the longer distance scale. But if $\Omega =1$ we have a challenge: why does some 90\%\ of the mass appear to have resisted clustering on scales $\lap 10h^{-1}$~Mpc? Astrophysical biasing scenarios postulate galaxy formation was inhibited by unhealty conditions in protovoids. But if the gross properties of galaxies were sensitive to environment, why do the galaxies that survived exhibit the striking environment-independent regularities of the Tully-Fisher relation and the fundamental planes? In some versions of the $\Omega = 1$ cold dark matter and mixed dark matter models galaxies are assembled at low redshifts. If so, perhaps the Carlberg velocity biasing effect can account for the low apparent $\Omega$ in the small-scale dynamical tests. Galaxy assembly at low redshift certainly could agree with the apparent immaturity of galaxies at $z\gap 0.7$, as indicated by the Butcher-Oemler effect and the alignment effect in radio galaxies. But if galaxy assembly were a recent phenomenon why do the Wolfe clouds at $z\sim 3$ look like young galaxies in an already well-developed intergalactic medium?
The central message of these dilemmas is that we have a considerable and growing collection of pieces to the puzzles of cosmogony. My quess is we have excellent prospects in the next few years for eliminating wrong pieces and discovering how some of the rest fit together.
