By coincidence, I ran across a recent paper discussing the "missing
baryon" problem:
https://doi.org/10.1002/asna.201713343

[[Mod. note -- I've fixed the slightly-garbled DOI URL. The article
itself is paywalled, but the preprint is on the arXiv:
https://arxiv.org/abs/1611.03722
-- jt]]

Don't overlook that Fig 1 double-counts some small but unknown amount
of the hot gas, as noted in the figure caption. In other words, the
combined red and green sectors should be a little smaller than shown.

An upcoming _Nature_ paper by the same lead author will suggest that
most or all of the missing baryons are in the form of very hot
intergalactic gas, detected as O VII. That is difficult but not
quite impossible to observe with current instruments. Future X-ray
observatories should give much better information.

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