A secret history is a revisionist retelling of history, which has previously been suppressed or ignored. This can be either real, such as books about the WW2 deception Operation Mincemeat and its role in the conquest of Sicily; this can be a fake pretending to be real, such as conspiracy theories about how the Council of Foreign Relations think-tank has been trying to form a one-world government; or a fictional story that shows what "really" happened during a known historical event or involving a historical figure, a common trope in science fiction and fantasy stories.
In the case of Daria fanfiction, a secret history is a story that purports to reveal the truth about "what really happened" during a certain canon event (or series of events), without changing the ostensible face of what was seen on the TV series. The story may make use of character viewpoints not examined in a certain episode, showing what occurred out of eyesight of the viewer that actually affected the story's plot and outcome.
Not all missing scenes count as secret histories. For instance, an original adventure that Daria Morgendorffer and Jane Lane have during the summer between their sophomore and junior years at Lawndale High would not necessarily count as a secret history, even if their adventure was unknown to anyone else. If it does not intersect with and explain something in canon, it isn't a secret history.
There are not many examples of secret histories in Daria fanworks. One is "Scarlett," which covers events in the first season of Daria from "Esteemsters" onward, particularly from "The Lab Brat" to "The Big House." Certain background characters appearing in the episodes are discovered to have been involved in a legendary magical war, and their actions have consequences appearing in later Daria episodes. (For instance, some background characters wrote scripts for Sick, Sad World episodes that Daria and Jane later viewed.) Another example, "A Hard Days' Night," presents the "truth" about the bizarre events that took place during the show's third season, especially during "Depth Takes a Holiday"; "The Shadow Education" shows the secret machinations of Lawndale's teachers during See Jane Run. "But in Her Heart a Cold December" looks into the "real" Angela Li and the reasons for her obsession with school security, seen through the events of the Korean War. Much of "Deus Jane" could be considered a secret history of the first four seasons of Daria, though it diverges from canon when covering the events of "Dye! Dye! My Darling."
A stunning ficlet by WacoKid from May 2005 illustrates the power of even a brief look behind the curtain of canon, in this case tearing away the veil of what was really happening during "I Don't." (FYI: "Hat trick" is explained here.) One of the false endings of Smileyfax's tale, "Box to the Future," creates a secret history in which Jake Morgendorffer and Andrew Landon must preserve causality when time-travelers start screwing up canon.
Daria fanfics have also played with the third definition of secret history and showed world history to be different: "Prayers for a SAINT" gives an altered version of real history in which a prototype nuclear-missile platform launched into space by the Soviet Union in 1987 (actually attempted) was instead the real thing, and an interceptor mission was launched by the United States (using a real-life satellite launch schedule) to stop it.
Artist Kemical Reaxion once drew what could be considered a "secret history" artwork, showing the "real reason" why Daria screwed up the tiger-striping job on Jane's hair in "Dye! Dye! My Darling." [Note: Link might work best if cut and pasted into the browser.]
- Secret history at Wikipedia