How to Spot a Black Hole in a Most Black Universe

As you get closer to a black hole, time starts to slow down. If your friend is right at the event horizon, you see them basically frozen in time. The first instance of a recorded black hole was in 1971 and we've discovered dozens since then. But how do you spot a black hole in a mostly black universe? Well, perhaps I should ask someone who's done exactly that. Dr James Nightingale recently found a supermassive black hole 33 billion times the mass of our son. He told me about the different ways you can go black hole spotting.