The phrase "white darkness" describes a paradoxical phenomenon central to the Antarctic landscape. In polar summer, when Sym visits, the sun never sets. That means it's always light, even at nighttime. "They call it the White Darkness," Sym says. "A window opaque with condensation. A cataract over the eyeball." It is the "presence of light without any of the usual complications—like being able to see" (20.38).
Such a study in contrasts is befitting of a book where things are never as they seem. Victor, who poses as a generous benefactor, is actually a murderer. Manfred poses as Victor's biggest fan, but he's actually stealing his money. And Sym feels unbearably awkward, even as many of her fellow travelers have been nursing a crush on her.