  Compared to purely atomic collisions, ultracold molecular collisions
potentially support a much larger number of Fano-Feshbach resonances due to the
enormous number of ro-vibrational states available. In fact, for alkali-metal
dimers we find that the resulting density of resonances cannot be resolved at
all, even on the sub-$\mu$K temperature scale of ultracold experiments. As a
result, all observables become averaged over many resonances and can
effectively be described by simpler, non-resonant scattering calculations. Two
particular examples are discussed: non-chemically reactive RbCs and chemically
reactive KRb. In the former case, the formation of a long-lived collision
complex may lead to the ejection of molecules from a trap. In the latter case,
chemical reactions broaden the resonances so much that they become
unobservable.
