At Space.com, some think we have some hints:
One possible solution to this cosmic quandary is that the baryons are somewhere out there, not lighting up as stars, not compact enough to make gravitational lenses, and not dense enough to absorb background light. The missing baryons could just be … floating around, minding their own business, not really associated with any particularly interesting object.
And in the larger universe, when you want to get away from the hustle and bustle of the galaxies, you go to the filaments — long, thin tendrils of gas that connect galaxies to their neighbors, like long stretches of empty highways between cities.
We know of the existence of these filaments through computer simulations, but measuring them is much harder, since they’re so thin and feeble.
But recent techniques are starting to open them up…
And that’s what we’re beginning to find: about half the baryons in our universe eschew big-city living, and prefer to live in the sleepy rural stretches between them
Paul Sutter, “Where did all the baryons go?” at Space.com
A really simple explanation like that, if it checks out, would be welcome for a change.