  We construct the gravitating mass of an isolated composite system on
asymptotically-flat spacetimes within conventional general relativity and
investigate when this quantity is well defined. For stationary spacetimes, this
quantity is known to exactly equal the physical (ADM) mass. However, it remains
an open question whether these two masses are equal in the absence of a
timelike Killing vector. This is especially apropos since our universe has an
`origin' and hence no such Killing vector. Further, if these masses failed to
agree then composite systems could behave as if they had a `dark component,'
whose gravitating mass would not equal the physical mass-energy present. The
existence of such an apparent discrepancy is indeed ubiquitous in galaxies and
galaxy clusters, though currently it is attributed to the presence of dark
matter. We conclude that the theoretical question of the relation between these
masses for dynamical spacetimes is ripe for attention.
