Excession

Excession is a science fiction novel by Iain M. Banks and the fourth published to feature The Culture. It is a large-scale space opera with a complex plot following many characters across large volumes of space.

Warning: Wikipedia contains spoilers

The book details the Culture's response to an "Outside Context Problem", the Excession of the title (essentially a Big Dumb Object, a perfect blackbody sphere that appears mysteriously on the edges of Culture space, is Mysterious and Ineffable for a while, and then disappears). (could probably add some stuff about the Affront here?)