  We develop the theory of radiative association of an atom and a diatomic
molecule within a close-coupling framework. We apply it to the formation of
H$_3^-$ after the low energy collision (below 0.5 eV) of H$_2$ with H$^-$.
Using recently obtained potential energy and permanent dipole moment surfaces
of H$_3^-$, we calculate the lowest rovibrational levels of the H$_3^-$
electronic ground state, and the cross section for the formation of H$_3^-$ by
radiative association between H$^-$ and ortho- and para-H$_2$. We discuss the
possibility for the H$_3^-$ ion to be formed and observed in the cold and dense
interstellar medium in an environment with a high ionization rate. Such an
observation would be a probe for the presence of H$^-$ in the interstellar
medium.
