Electric Force
The answer comes from the fact that, while gravity is only attractive, electricity can be attractive AND repulsive.
Since positive and negative charges tend to attract, they will tend to come together and cancel one another out. If a third charge is in the area of the two that have come together, it will be attracted to one, but repulsed from the other. If the first two charges are equal, the attraction and repulsion on the third will balance out, just as if the charges weren’t there!