The first paper I looked at (Paraficz et al. 2012 http://arxiv.org/abs/1209.0384 ) explains that the hot gas mass is determined from X-ray observations (the X-ray flux from an optically thin gas depends on the square of the gas density multiplied by its volume - if you can then estimate the volume you get the density and also the gas mass) and masses of individual galaxies are estimated by modelling their luminosities through Faber-Jackson or (for spirals) Tully-Fisher scaling relations.

The assumption is that this contains all the baryonic mass (though the galaxy masses will also contain a dark matter component). It is on this basis that it is claimed that the X-ray emitting gas contains *a similar* amount of mass to that associated with individual galaxies. *If* those galaxies have non-baryonic dark matter halos that dominate their total mass (which seems likely unless they have extraordinarily low luminosity to mass ratios) then I think this leads to the claim that about 90 per cent of the baryonic mass is in the X-ray emitting gas.

Gravitational lensing then reveals that the galaxies plus hot gas only represents 20 per cent of the total cluster mass (9 per cent in hot gas, 11 per cent in galaxies) and thus that 89 per cent of the total mass is not in galaxies.