  We present the theory of radiative association of atoms and molecules, and we
apply it to the (H$_2$-H$^-$) van der Waals complex. We discuss the possibility
for the H$_3^-$ ion to be formed in the interstellar medium in an environment
with abundant ionized molecular hydrogen H$_2^+$. The observation of H$_3^-$
would also be a probe for the presence of H$^-$ in the interstellar medium. By
computing the electronic structure of the H$_3^-$ ion, we determine its dipole
moment, bound states, rotational constants, predissociated vibrational
resonances and their lifetimes, and suggest a way to detect the ion in the
interstellar medium.
