We propose an experiment using optically trapped and cooled dielectric micro-spheres for the detection of short-range forces. The center-of-mass motion of a microsphere trapped in vacuum can experience extremely low dissipation and quality factors of 10(12), leading to yoctonewton force sensitivity. Trapping the sphere in an optical field enables positioning at less than 1 μm from a surface, a regime where exotic new forces may exist. We expect that the proposed system could advance the search for non-Newtonian gravity forces via an enhanced sensitivity of 10(5)-10(7) over current experiments at the 1 μm length scale. Moreover, our system may be useful for characterizing other short-range physics such as Casimir forces.