How this works is that the CMB
photons can't travel very far before they
Thomson scatter
off an electron. The two are therefore tightly-coupled together and
behave like a nearly perfect fluid. Likewise the electrons
can't travel very far from a baryon before
electromagnetic
or Coulomb interactions force the charged
baryons to follow:
[Note: This picture isn't quite fair since there
are actually substantially more CMB photons
than baryons or electrons in the universe.
Despite the inertial mass of the baryons, photons actually dominate the
dynamics of the plasma. The fluid is a relativistic
fluid.]
