  By means of extensive galactic modeling we study the implications of the more
than eighty microlensing events that have now been observed for the composition
of the dark halo of the Galaxy, as well as for other properties of the Galaxy.
We take the Galaxy to be comprised of luminous and dark disk components, a
bulge, and a dark halo consisting of both MACHOs and cold dark matter with each
component being described by several observationally motivated parameters. We
pare down an initial model space of millions of galactic models to viable
models, those which are consistent with the observational data, including
rotation curve, local projected mass density, and microlensing rates toward the
LMC and bulge. On the basis of a conservative, minimal set of observational
constraints an all-MACHO halo cannot yet be excluded, although in most viable
models of the Galaxy the halo MACHO fraction is between 0\% and 30\%,
consistent with expectations for a universe whose primary component is cold
dark matter. An all-MACHO halo is required to be light, and when data on the
local escape velocity and satellite-galaxy proper motions, which probe the
extent of the dark halo, are taken into account, models which have a high MACHO
mass fraction are ruled out. We also explore the possibility that there are no
MACHOs in the halo. Finally, we point out several important tests that could
definitively exclude an all-MACHO e.g., optical depth for microlensing which is
less than $1.5\times 10^{-7}$ toward the LMC or greater than $3\times 10^{-6}$
toward the bulge.
