Would a charge imbalance act like dark energy? I realize that there are theoretical reasons to reject the idea that the charges on electrons and protons may not be exactly equal and opposite; and I am not suggesting that they're not.
Edited 12/20/18: However, I would like to know: if there were a very, very tiny imbalance between the electron's charge and the proton's charge (on the order of one part in $10^{36}$ or less), would it result in a cosmic expansion that resembles the expansion attributed to dark energy?  
The rationale is that all atoms would have a very slight net charge of the same sign if there were such an imbalance; and as a result there would be a very slight net repulsive electrostatic force between all nominally neutral atoms.  If the imbalance were small enough, atoms should still form, gravity should be sufficient to bind most (nominally neutral) matter together on most scales, and most other phenomena should be as currently observed, but it seems that at cosmological distances there might be some observable effects.
 A: I don't think this idea works, for the simple reason that the electromagnetic and gravitational forces scale the exact same way: they are both proportional to their respective 'charge' and inverse square. 
Since your idea implies most apparently neutral objects would have roughly the same charge to mass ratio (since it depends on just the electron density), if the repulsion effect were strong enough to beat gravity at cosmological scales, it would also beat gravity on everyday scales, because the ratio of the two would remain exactly the same. But we're not repelled from the Earth.
A: I don't think this idea works. Two hand-wavy reasons:


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*This would predict that everything would be repelled from everything else on cosmological scales, which isn't true. Most things are receding from us, but not everything (e.g. Andromeda is approaching the Milky Way).

*If there was an imbalance in electric charge in galaxies, why wouldn't free electrons be attracted to galaxies and neutralize it? For this same reason, electrically-charged black holes aren't as astronomically significant as their electrically-neutral black holes.
