As long as stellar black holes were the only kind of black holes for which science could find even indirect evidence, the universe seemed a far less scary place than it does today. After all, stellar black holes did not appear to pose any major short- or long-term danger to the universe as a whole or to the existence of life within it. True, when a giant star collapses to form a black hole, any living things inhabiting the planets or moons of that solar system will first be fried and then frozen. No life of any kind will be able to survive for very long. However, these lethal effects would remain localized to that system. This is because the distances separating most stars are immense- about four to seven light-years, or...