  Dark matter, proposed decades ago as a speculative component of the universe,
is now known to be the vital ingredient in the cosmos, eight times more
abundant than ordinary matter, one quarter of the total energy density and the
component which has controlled the growth of structure in the universe. Its
nature remains a mystery, but, assuming it is comprised of weakly interacting
sub-atomic particles, is consistent with large scale cosmic structure. However,
recent analyses of structure on galactic and sub-galactic scales have suggested
discrepancies and stimulated numerous alternative proposals. We discuss how
studies of the density, demography, history and environment of smaller scale
structures may distinguish among these possibilities and shed new light on the
nature of dark matter.
