H I column density profiles of dark galaxies for the HESTIA simulations (left) and the NIVARIA-LG simulations (right), coloured by H I mass. The dotted-dashed grey line represents the minimum H I column density value that FAST can reach, which is N_H I = 2 × 10¹⁷ cm⁻². The coloured shaded area in each panel shows the softening length corresponding to each simulation. A smaller number of dark galaxies could be detected by the less sensitive ALFALFA Survey, whose sensitivity is N_H I ≳ 2 × 10¹⁸ cm⁻².