what is the dark thing

The Dark Thing

While it is not a character in a physical sense, the Dark Thing personifies evil in the universe. It is described as a black force that blots out the light of stars and planets in the universe. Stars can combat the Dark Thing, but in order to defeat it, they will necessarily lose their own lives.

The "dark thing" also referred to in the movie as "It" is a dark force of evil, while not a character, it is still a great force which plays a great role in A Wrinkle in Time. "It" is simply a dark force that attempts to get rid of love and happiness throughout the universe.