[whswhs]

If you want to think in terms of actual physics—not a requirement, as others here have said—it isn't necessary to look either for new fundamental forces, or for yet more elementary particles. A different direction has been emerging over the past half century or so: the physics of nonequilibrium situations such as systems with a steady influx of free energy (for example, planets illuminated by a sun), of systems with sensitive dependence on initial conditions such as weather, and of emergent self-organized complexity from flows of energy. This last might even provide a way of tying the Force into conventional physics, with a bit of handwaving; the life that "creates it; makes it grow" is one of the quintessential examples of emergent complexity, or so it appears. The traditional mathematics either of small numbers of particles (the three-body problem is already challenging!) or of random collections of particles (Asimov's model for the Foundation stories) can't cope with this; you need different sorts of math.