Missing Baryon Problem Solved

Connor et al. assess that about 3/4 of all baryons are in the intergalactic medium (IGM), give or take 10% – the side bars illustrate the range of uncertainty. Many of the remaining baryons are in other forms of space plasma associated with but not in galaxies: the intracluster medium (ICM) of rich clusters, the intragroup medium (IGroupM) of smaller groups, and the circumgalactic medium (CGM) associated with individual galaxies. All the stars in all the galaxies add up to less than 10%, and the cold (non-ionized) atomic and molecular gas in galaxies comprise about 1% of the baryons.

For a long time at least half to a third of the ordinary atoms that other observations and Big Bang Nucleosynthesis predicted exist hadn't been found. (And, to be clear, this "missing baryon problem" was separate and distinct from the "dark matter" problem.)

Now, they are all accounted for. The missing ones were in the intergalactic medium (i.e. in the deep space between galaxies at a density of about one hydrogen atom per cubic meter). 

Stacy McGaugh at his blog Triton Station explains how this happened. The money chart is above.