  We present two-photon photoassociation to the least-bound vibrational level
of the X$^1\Sigma_g^+$ 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.
