The nature of dark gamma-ray bursts.

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'. 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. We 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. [ABSTRACT FROM AUTHOR]