We compare the properties of optically dark GRBs , defined by the optical-to-X-ray spectral index \beta _ { OX } < 0.5 , and normal ones discovered by the Swift satellite before the year 2008 in a statistical way , using data collected from the literature and online databases .
Our sample include 200 long bursts , 19 short bursts , and 10 with measured high redshifts ( z \gtrsim 4 ) .
The ratio of dark bursts is found to be \sim 10 - 20 \% , and is similar between long bursts , short ones , and the high- z sub-sample .
The result for long bursts is consistent with both the pre- Swift sample and studies by other authors on smaller Swift samples .
The existence of dark short GRBs is pointed out for the first time .
The X-ray derived hydrogen column densities of dark GRBs clearly prefer large values compared with those of normal bursts .
This supports the dust extinction scenario as the main cause of dark GRBs .
Other possibilities like very high redshifts and non-standard emission mechanisms are less likely although not fully excluded .