  We propose and experimentally demonstrate that the trihydrogen cation
(H$_3^+$) can be produced via single photoionization of the molecular hydrogen
dimer (H$_4$). Using near-infrared, femtosecond laser pulses and coincidence
momentum imaging, we find that the dominant channel after single ionization of
the dimer is the ejection of a hydrogen atom within a few hundred femtoseconds,
leaving an H3+ cation behind. The formation mechanism is supported and well
reproduced by an ab-initio molecular dynamics simulation. This is a new pathway
of H$_3^+$ formation from ultracold hydrogen gas that may help explain the
unexpected high abundance of H$_3^+$ in the interstellar medium in the
universe.
