> When talking about the aerodynamic center, Where is the force?
A pitching moment is a torque. The interesting thing about a torque is that it's not a unique force acting at unique distance.
For example if I tell you the torque is 12 units you have no way of knowing if that's created by a 12 unit force at a 1 unit distance or by a 1 unit force at a 12 unit distance or any other combination (2 x 6 or 3 x 4 etc). It's all the same.
Where does the pitching moment come from? Perhaps think of it like this.. On a wing section each part of the chord contributes a bit to the pitching moment. A little bit of lift generated by a patch near the trailing edge on the top surface might contribute a pitch down, a little bit near the leading edge a pitch up etc. To work out the overall pitching moment the lot has in effect to be summed or integrated.
Most conventional sections have a negative (nose down). Consider what you need for a non swept tailless aircraft - you need a positive pitching moment. To turn a conventional section into one suitable for a tailless aircraft the trailing edge it typically "bent up". That modifies the overall pitching moment for the wing and provides stability (unfortunately it also reduces lift).