  We present here a comprehensive study of the optical/near-infrared (IR) upper
limits for gamma-ray bursts that have an X-ray afterglow. We have extrapolated
the X-ray afterglows to optical wavelengths based on the physics of the
fireball blast wave model (e.g. Rees & Meszaors 1992; Meszaros & Rees 1997),
and compared these results with optical upper limits for a large sample of
bursts. We find a small set of only three bursts out of a sample of 20 for
which the upper limits are not compatible with their X-ray afterglow properties
within the context of any blast wave model. This sparse sample does not allow
us to conclusively determine the cause of this optical/near-IR deficit.
Extinction in the host galaxy is a likely cause, but high redshifts and
different afterglow mechanisms might also explain the deficit in some cases. We
note that the three bursts appear to have higher than average gamma-ray peak
fluxes. In a magnitude versus time diagram the bursts are separated from the
majority of bursts with a detected optical/near-IR afterglow. However, two GRBs
with an optical afterglow (one of which is highly reddened) also fall in this
region with dark bursts, making it likely that dark bursts are at the faint end
of the set of optically detected bursts, and therefore the dark bursts likely
form a continuum with the bursts with a detected optical afterglow. Our work
provides a useful diagnostic tool for follow-up observations for potentially
dark bursts; applied to the events detected with the Swift satellite, it will
significantly increase our sample of truly dark bursts and shed light upon
their nature.
