Heat transfer between baryons and millicharged dark matter has been invoked as a possible explanation for the anomalous 21-cm absorption signal seen by EDGES .
Prior work has shown that the solution requires that millicharged particles make up only a fraction ( m _ { \chi } / { MeV } ) 0.0115 \% \lesssim f \lesssim 0.4 \% of the dark matter and that their mass m _ { \chi } and charge q _ { \chi } have values 0.1 \lesssim ( m _ { \chi } / { MeV } ) \lesssim 10 and 10 ^ { -6 } \lesssim ( q _ { \chi } / e ) \lesssim 10 ^ { -4 } .
Here we show that such particles come into chemical equilibrium before recombination , and so are subject to a constraint on the effective number N _ { eff } of relativistic degrees of freedom , which we update using Planck 2018 data .
We moreover determine the precise relic abundance f that results for a given mass m _ { \chi } and charge q _ { \chi } and incorporate this abundance into the constraints on the millicharged-dark-matter solution to EDGES .
With these two results , the solution is ruled out if the relic abundance is set by freeze-out .