“Silhouettes are reductions,” the artist Kara Walker said in 2014, “and racial stereotypes are also reductions of actual human beings.” Walker’s panoramic silhouettes, which confront America’s appalling history of racism and slavery, have been haunting museumgoers since 1994, when she first burst upon the art scene. Walker critiques unjust power structures in straightforward, poignant, and sometimes violent imagery. In this retrospective, 80 works are on view. —E.C.