The Schur complement structure with respect to 2-by-2 principal submatrices, in hollow, symmetric nonnegative matrices is investigated, with an emphasis on such matrices that have only two nonpositive eigenvalues. It is shown that a wide family of such Schur complements simply follows a unique and surprising structure that can be fully described in a graph theoretical language, and is predictable from the entries. For larger numbers of nonpositive eigenvalues, conjectures regarding connections to polyhedra are also presented, and proved in a special case. Relations to copositive matrices and Morishima matrices are described as well.