We use the isotropy of the Cosmic Microwave Background to place stringent constraints on a possible electrical charge asymmetry of the universe .
We find the excess charge per baryon to be q _ { e - p } < 10 ^ { -26 } e 
in the case of a uniform distribution of charge , where e is the charge of the electron .
If the charge asymmetry is inhomogeneous , the constraints will depend on the spectral index , n , of the induced magnetic field and range from q _ { e - p } < 5 \times 10 ^ { -20 } e ( n = -2 ) to q _ { e - p } < 2 \times 10 ^ { -26 } e ( n \geq 2 ) .
If one could further assume that the charge asymmetries of individual particle species are not anti-correlated so as to cancel , this would imply , for photons , q _ { \gamma } < 10 ^ { -35 } e ; for neutrinos , q _ { \nu } < 4 \times 10 ^ { -35 } e ; and for heavy ( light ) dark matter particles q _ { dm } < 4 \times 10 ^ { -24 } e 
( q _ { dm } < 4 \times 10 ^ { -30 } e ) .