It means the total mass of luminous and non-luminous baryonic matter in the (rich) galaxy clusters. Rich clusters, having 50 or more members, account for only 15% of the critical mass fraction (27% of total energy budget-- .15 X .27 = 4%) of the universe-- the rest is in poor (sparse) clusters and the IGM (loose gas). The phenomenon that the authors are attempting to explain is that in rich clusters, instead of each galaxy having its own gas and DM halo, the cluster is enveloped in a common halo. The observed temperature of the gas component is in the tens of millions of degrees K; hence, the production of X-rays.

I always keep a wary eye on the "fine-tuning" aspect of this sort of theory-- given enough free parameters, they can 'tune in' any sort of observed phenomenon. It may be that the universe is that complicated, but I doubt it.

Prof. Karachentsev's work, on the other hand, has the virtue of simplicity-- and more to the point, it has predictive power! It is much more satisfying somehow to predict the range and distribution of a universal phenomenon in advance, than to explain each individual instance on a case-by-case basis, citing possibility as the justification for each choice.

It's more than an aesthetic choice. I like to tell the story about the caterpillar who, when he was asked how he could possibly manage to control so many legs at once, stopped to think about it-- and never moved again. Theories which are too self-involved, with too many levers and wheels, bells and whistles, are not useful. For instance; M-Theory, which may contain the complete truth, contains a lot else, besides. It's "not even wrong", yet.

I don't say that Dr. K has the final answer. I think that there are (several) more types of DM which will become explicable when Big Bang Theory and particle physics become reconciled.

Sorry about the rant--I got started thinking about my decision-making process and ...

__________________
Ignoramus et ignorabamus.-- Reymond

Wir mussen wissen. Wir werden wissen.--Hilbert

Pick one.