Libby Day's entire past is a dark place where the memories of her slaughtered mother and sisters reside. She tells us in chapter one, "I've labeled the memories as if they were a particularly dangerous region: Darkplace" (1.56). And when Lyle later asks her, "You really don't think about that night" (1.106), Libby thinks: "Darkplace" (1.107).
Libby doesn't go into a shaking spiral of depression or suffer a panic attack or anything like that. It's almost like the Darkplace doesn't exist. It's a black square in her memory. A couple more times during the novel, Libby casually uses the term—"My mother's banshee screams. Darkplace" (5.9)—but we're not sure if she can't remember it, or if she's actively blocking it out. Either way, she doesn't want to hear those screams again, and we can't blame her.
At the very end of the novel, Libby is happy to "just listen to the quiet […] away from Darkplace" (42.14). She now knows what happened to her family, and that her brother didn't kill them. It'll never be easy to deal with her past, but this revelation makes things a little simpler. Libby doesn't have to go back to Darkplace over and over again trying to bring light to it. She can finally move on.