We present two-photon photoassociation to the least-bound vibrational level of the X$^1Sigma_g^+$ electronic ground state of the $^{86}$Sr$_2$ dimer and measure a binding energy of $E_b=-83.00(7)(20)$,kHz. Because of the very small binding energy, this is a halo state corresponding to the scattering resonance for two $^{86}$Sr atoms at low temperature. The measured binding energy, combined with universal theory for a very weakly bound state on a potential that asymptotes to a van der Waals form, is used to determine an $s$-wave scattering length $a=810.6(12)$,$a_0$, which is consistent with, but substantially more accurate than the previously determined $a=798(12),a_0$ found from mass-scaling and precision spectroscopy of other Sr isotopes. For the intermediate state, we use a bound level on the metastable $^1S_0-{^3P_1}$ potential. Large sensitivity of the dimer binding energy to light near-resonant with the bound-bound transition to the intermediate state suggests that $^{86}$Sr has great promise for manipulating atom interactions optically and probing naturally occurring Efimov states.
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