What is the filler market, you might ask? According to Marie E. Cecchini on her blog, Writing-World.com, Webster defines "filler" as "material used to fill extra space in a column or on a page of a newspaper or magazine." She writes: “Fillers can be written for children or adults, and topics for them run the gamut from parenting, health, and weight loss, to redecorating your room, dealing with finance concerns, working with technology, and managing stress.”
Children’s book author, Kathleen Muldoon’s article, “Short, Sweet and Sometimes Profitable – Writing for the Filler Market,” addresses this topic on the Institute of Children’s Literature blog. The author of numerous children’s books, Muldoon writes that fillers come in many forms such as "short anecdotes, notable quotes, and interesting bits of trivia." She gives examples of other types of fillers. Maybe you have a short anecdote you might want to submit to a magazine. Look through those files and I'll bet you'll find one.