[@riess98; @perlmutter99] elevated the CDM model, in its variant with a cosmological constant ($\Lambda$CDM), to the status of a standard paradigm. At the same time improving numerical techniques and faster computers have enabled detailed simulation of the formation and evolution of the galaxy population within this paradigm throughout a significant fraction of the observable Universe [@mr_nature]. Nevertheless, direct observational evidence for halos as extended as the paradigm predicts around galaxies like our own has so far come only from statistical analyses of the dynamics of satellite galaxies [e.g. @zaritsky97; @prada03] and of the gravitational lensing of background galaxies [e.g. @seljak02; @mandelbaum06] based on large samples of field spirals.

The earliest observational indication that the effective mass of the Milky Way must be much larger than its stellar mass came from the Timing Argument (hereafter TA) of @kw59. These authors noted that the Local Group is dominated by the two big spirals, and that these are currently approaching each other at about $100{{\rm \,km\,s^{-1}}}$. (The next most luminous galaxy is M33 which is probably about a factor of $10$ less massive than M31 and the Galaxy.) This reversal of the overall cosmic expansion must have been generated by gravitational forces, and 