It is a standard result of General Relativity that time slows down near a black hole. Thus one can perch a spacecraft near a black hole and voila, a century of time for the spacecraft will be equivalent to millions of years outside. Also matter continues to fall at energetic rates at the center of massive black holes at the Galactic center. Thus a fleet of spacecraft can over indefinitely near the event horizon siphoning energy from the infallible matter to power their space stations there. It would be logical for any sufficiently advanced civilization to keep such a fleet containing thousands of individuals near a black hole to protect the species from extinction from unforeseen events over millions of years in the outside. Small groups would repeatedly make forays in the outer regions to reseed or explore nearby areas etc. So if intelligent aliens are there, they should have made such outposts near energetic black holes. That's where one should look.