Symbolism, Imagery, Allegory
The Space Between
The Fitzgeralds are a family that has had their eye on the sky for a long time. Anna is named after Andromeda, the galaxy (and, kind of, the myth), and both Anna and Brian spend a lot of time stargazing from the roof of the fire station. Spoiler alert: While everyone's worrying about whether Kate will die, the sky—and the connections that get made to Anna—are pretty much foreshadowing that she's actually the one who'll bite in the end. Anna's the one destined for the great beyond, not her sister.
Space analogies are worked into the story often. Early on, we're treated to a definition of black holes, which are described as "so heavy they absorb everything, even light, right into their center" (1.1.29), which is a way we'd describe the Fitzgerald family—a family so dark and dysfunctional, we'd be afraid to visit because we might not ever be able to leave their screwed-up little universe. And you know whose light they're really sucking up? Anna's. After all, they only created her as an antidote to Kate's troubles—girl's never been seen for her own worth.
Another notable incident occurs when Brian and Anna bond while watching the Perseids meteor shower. Anna wonders if meteor showers happen every night, and this is just the first one she's seen: "Do all the wonderful things happen when we are not aware of them?" (4.7.83), she asks. What she's talking about here is how the world keeps spinning, even when she's asleep. And it will still go on after Kate is dead, too—or, you know, after Anna's dead, as the case winds up being.
To explain her life to herself, Anna creates her own mythology. It's a story in which the moon starves itself and pokes holes in the sky, creating the stars. The sun gets jealous of the stars and tries to take them. Some of them go to the sun's side of the planet, but they can't be seen (Anna's invisibility complex is shining through), so the moon carves "blocks of sorrow" (6.2.87) into people—which is another good way to describe the Fitzgerald family: "blocks of sorrow" who are largely invisible because Sara and Kate shine so brightly.
Space, then, is a symbol for Anna and the bummer that is her life.