What is the dreaded it in madeline A Wrinkle Time?

In Madeleine L'Engle's classic science fiction novel "A Wrinkle in Time," there are various dangers and challenges faced by the main characters, but there is no specific entity mentioned throughout the book which goes by the term "dreaded it." Each threat faced by the characters has distinct forms and descriptions in the story. Could you please mention or describe the context where "dreaded it" seems to be an element?