  We observe the dynamic formation of $Cs_2$ molecules near Feshbach resonances
in a cold sample of atomic cesium using an external probe beam. This method is
300 times more sensitive than previous atomic collision rate methods, and
allows us to detect more than 20 weakly-coupled molecular states, with
collisional formation cross sections as small as $\sigma =3\times
10^{-16}$cm$^2$. We propose a model to describe the atom-molecule coupling, and
estimate that more than $2 \times 10^5$ $Cs_2$ molecules coexist in dynamical
equilibrium with $10^8$ $Cs$ atoms in our trap for several seconds.
