Relation between the stellar mass and the enclosed dark matter mass within R_HI. The upper panel shows the median and the 1σ scatter (16th to 84th percentile) for the ALFALFA sample galaxies (blue solid), the mock observations (TNG100-mock, red solid) and the true value (TNG100-true, orange dash-dotted) of the TNG100 sample, the counterpart of the sample from the TNG100-dark simulation (pink dashed), and a ‘dark matter’ mass derived by removing the true baryonic mass of the TNG100 from the mock dynamical mass of the TNG100-mock (TNG100-mock*, red dotted). The lower panel presents the relative change of the median values compared to that of the TNG100-dark line in the same colour scheme and line style. We also indicate the errors for the median values of the ALFALFA sample and TNG100-mock with their corresponding error bars.