9. Who Killed Barry's Mother? - The Flash
Despite The Flash being centered around the murder of Barry Allen's (Ezra Miller) mother Nora (Maribel Verdú), the film never actually reveals the identity of the person who fatally stabbed her in her own kitchen.
You can argue that the killer isn't terribly important in the grand scheme of things, yet in both the comics and the CW-produced Flash TV series, Barry's mother was killed by supervillain Eobard Thawne aka Reverse Flash aka Professor Zoom.
However, we get not even a hint of Reverse Flash's presence in this film, leaving the culprit of Nora's murder totally up in the air.
That said, in post-release interviews director Andy Muschietti did heavily imply that the movie followed in the footsteps of the comics and TV series, and indeed, Nora here was also killed by Reverse Flash.
All the same, we never see it happen on-screen nor is it even remotely hinted at, which combined with a planned sequel clearly not happening, means we're never going to get concrete confirmation.