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

A cosmologically consistent millicharged dark matter solution to the EDGES anomaly of possible string theory origin