  All stellar mass black holes have hitherto been identified by X-rays emitted
by gas that is accreting onto the black hole from a companion star. These
systems are all binaries with black holes below 30 M$_{\odot}$$^{1-4}$. Theory
predicts, however, that X-ray emitting systems form a minority of the total
population of star-black hole binaries$^{5,6}$. When the black hole is not
accreting gas, it can be found through radial velocity measurements of the
motion of the companion star. Here we report radial velocity measurements of a
Galactic star, LB-1, which is a B-type star, taken over two years. We find that
the motion of the B-star and an accompanying H$\alpha$ emission line require
the presence of a dark companion with a mass of $68^{+11}_{-13}$ M$_{\odot}$,
which can only be a black hole. The long orbital period of 78.9 days shows that
this is a wide binary system. The gravitational wave experiments have detected
similarly massive black holes$^{7,8}$, but forming such massive ones in a
high-metallicity environment would be extremely challenging to current stellar
evolution theories$^{9-11}$.
