The weakness of gravity compared to the other subatomic forces is a real mystery. While nobody knows the answer, one credible solution is that gravity has access to more spatial dimensions than the other three known forces. In this video, Fermilab's Dr. Don Lincoln describes this idea, with the help of some very urbane characters. See: Big Mysteries: The Extra Dimensions
The extra dimensions of string theory which were originally viewed as a source of embarrassment for the theory, have proven to be instrumental in resolving a number of puzzles associated with 3+1 dimensional physics. I discuss examples of this in the context of black holes, gauge theory and particle phenomenology. See: Strings and the Magic of Extra Dimensions - Cumrun Vafa
(link now dead)Here’s an analogy to understand this: imagine that our universe is a two-dimensional pool table, which you look down on from the third spatial dimension. When the billiard balls collide on the table, they scatter into new trajectories across the surface. But we also hear the click of sound as they impact: that’s collision energy being radiated into a third dimension above and beyond the surface. In this picture, the billiard balls are like protons and neutrons, and the sound wave behaves like the graviton.
- Are There Extra Dimensions?(Number 3)