Read this introduction to a book. In outer space, mysterious entities called black holes seem up to no good. From the headlines, you’d think black holes were beasts with endless appetites, lying in wait for the next meal. By some reports they are “runaway,” out-of-control “predators” that “feed” on galaxies, only to “belch” and “spit out” what they don’t eat. . . . In short, they have a nasty reputation for being monsters “gone mad.” But you know what? A black hole isn’t a monster. It’s not even alive. . . . It has no dark, destructive desires. . . . A black hole is NOT a hole—at least not the kind you can dig in the ground or poke your finger through. . . . If a black hole is not a hole, then what in the universe is it? –A Black Hole Is Not a Hole, Carolyn Cinami DeCristofano Based on this introduction, what is the main topic of this book?