  Analysis of EDGES data shows an absorption signal of the redshifted 21-cm
line of atomic hydrogen at $z\sim 17$ which is stronger than expected from the
standard $\Lambda$CDM model. The absorption signal interpreted as brightness
temperature $T_{21}$ of the 21-cm line gives an amplitude of
$-$500$_{-500}^{+200}$ mK at 99\% C.L. which is a 3.8$\sigma$ deviation from
what the standard $\Lambda$CDM cosmology gives. We present a particle physics
model for the baryon cooling where a fraction of the dark matter resides in the
hidden sector with a $U(1)$ gauge symmetry and a Stueckelberg mechanism
operates mixing the visible and the hidden sectors with the hidden sector
consisting of dark Dirac fermions and dark photons. The Stueckelberg mass
mixing mechanism automatically generates a millicharge for the hidden sector
dark fermions providing a theoretical basis for using millicharged dark matter
to produce the desired cooling of baryons seen by EDGES by scattering from
millicharged dark matter. We compute the relic density of the millicharged dark
matter by solving a set of coupled equations for the dark fermion and dark
photon yields and for the temperature ratio of the hidden sector and the
visible sector heat baths. For the analysis of baryon cooling, we analyze the
evolution equations for the temperatures of baryons and millicharged dark
matter as a function of the redshift. We exhibit regions of the parameter space
which allow consistency with the EDGES data. We note that the Stueckelberg
mechanism arises naturally in strings and the existence of a millicharge would
point to its string origin.
