To make an analogy, imagine you saw the following:

A person suddenly grabs at some invisible thing, and he is driven backwards by the impact. From this, you can guess that he caught an invisible basketball. Even though you can't see the basketball, you can easily see the basketball's effect on the player.

It turns out that all interactions (aka forces) which affect matter particles are due to an exchange of force carrier particles. In the basketball analogy, the players are the matter particles who toss a basketball which is the force carrier particle. What we commonly call "forces" are the effect of the force carrier particles on matter particles.
A few rules to apply to force carriers: