  We report optical trapping of laser-cooled molecules at sufficient density to
observe molecule-molecule collisions for the first time in a bulk gas. SrF
molecules from a red-detuned magneto-optical trap (MOT) are compressed and
cooled in a blue-detuned MOT. Roughly 30% of these molecules are loaded into an
optical dipole trap with peak number density $n_0 \approx 3\times 10^{10}
\text{ cm}^{-3}$ and temperature $T\approx40$ $\mu$K. We observe two-body loss
with rate coefficient $\beta = 2.7^{+1.2}_{-0.8}\times 10^{-10} \text{ cm}^3
\text{ s}^{-1}$. Achieving this density and temperature opens a path to
evaporative cooling towards quantum degeneracy of laser-cooled molecules.
