  Context: Thirteen years after the discovery of the first afterglows, the
nature of dark gamma-ray bursts (GRB) still eludes explanation: while each
ng-duration GRB typically has an X-ray afterglow, optical/NIR emission is only
seen for 40-60% of them. Aim: Here we use the afterglow detection statistics of
the systematic follow-up observations performed with GROND since mid-2007 in
order to derive the fraction of "dark bursts" according to different methods,
and to distinguish between various scenarios for "dark bursts". Method:
Observations were performed with the 7-channel "Gamma-Ray Optical and
Near-infrared Detector" (GROND) at the 2.2m MPI/ESO telescope. We used the
afterglow detection rate in dependence on the delay time between GRB and the
first GROND exposure. Results: For long-duration Swift bursts with a detected
X-ray afterglow, we achieve a 90% (35/39) detection rate of optical/NIR
afterglows whenever our observations started within less than 240\,min after
the burst. Complementing our GROND data with Swift/XRT spectra we construct
broad-band spectral energy distributions and derive rest-frame extinctions. e
detect 25-40% "dark bursts", depending on the definition used. The faint
optical afterglow emission of "dark bursts" is mainly due to a combination of
two contributing factors: (i) moderate intrinsic extinction at moderate
redshifts, and (ii) about 22% of "dark" bursts at redshift $>$5.
