The earth is partially a dark planet in A Wrinkle in Time. The children learn this when the Happy Medium shows them in her crystal ball that a dark shadow hovers on the earth. What keeps the evil forces (The Black Thing) threatening earth at bay are the good people, chiefly religious leaders, creative people and philosophers. In contrast to earth, a fully dark planet, like Camazotz, is one that has been completely overtaken by The Black Thing. Other worlds, like the planet Uriel that the children visit, have not been invaded The Black Thing and are filled with love.
The idea that earth is a battleground between good and evil and partially covered in darkness accords with traditional Christian theology, which sees the earth as "fallen," that is, invaded by evil, but also "saved" by the presence of figures like Jesus Christ.