A slight inaccuracy
I have not read the paper yet, but given that it is about Sunyaev-Zeldovich effect and gravitational lensing in galaxy *clusters* I suppose what is "weighed" here is the clusters - the largest structures in the Universe - and not the constituent galaxies. There is more intergalactic matter (luminous or dark) in clusters than is "sitting in galaxies" as the article says.
I assume that the (surprisingly) inaccurate title in Nature influenced the Reg - the contents of the Nature piece clearly talk about "weighing clusters".