The Force is a cosmic soup created by all living things.
All of us beings are its ingredients. Even as we absorb its flavors, we can in turn influence the flavor of the soup around us.
The light side of the Force are those flavors which act in harmony with the whole; the dark side refers to flavors which are in conflict with it.
To be in harmony, a flavor must be temperate so as not to overpower the taste of the soup.
Dark side flavors have no such restriction. This is why the dark side may at first appear to be stronger than the light.
But look closer, and you sill see that the light side is better supported by the host of other flavors in the galaxy.
The only hope that the dark side has of succeeding is to overpower and corrupt the flavor of the entire soup.