The graviational force is the weakest of all forces. It is, however, the force we are most familiar with. As far as we know, there is only one kind of gravitational charge. If you have two particles, their electric charges may cancel each other out, but their gravitational charge will always add up.

In particle physics, where scientists work with very small groups of particles, it is impossible to feel any effect of the gravitational force. Even at the scale of humans, made of zillions of particles, there is no way we can feel the gravitational force between us. We can feel the gravitational force of the earth however: This is the force that makes us fall back on the ground when we jump into the air.

So far, there is no way to describe the gravivational force through the standard model. To explain gravity, you need the theory of relativity . Scientists are working very hard on bringing the standard model and relativity together. One such attempt is the string theory .

Another open question is whether antimatter has anti gravity. So far there haven't been made big enough amounts of antimatter to answer this question.

about gravitation, highly recommended!

About production of antimatter in Cern.

About Gravitation in German. (Gravitation)