Mysteries

In The Big Bang Theory, there have been several mysteries. These mysteries include what had happened to several recurring characters after disappearing without any reason or when any moments that happened in the show which fans and viewers don't know. These mysteries also include the entire main cast, both current and former (Leonard, Sheldon, Howard, Raj, Penny, Bernadette, Amy, Stuart, Leslie, and Emily), their families.

Main characters

 * What is Penny's last name?
 * Is Amy an only child?

Family Members

 * Is Penny's sister's husband alive or dead?
 * Why haven't Amy's mother and Bernadette's mother made anymore appearances?
 * Why did it take too long for Sheldon's grandmother, Leonard's father and Penny's mother and brother to appear?
 * Why did Howard's father abandon him and his mother when Howard was 11?
 * What happened to Howard's half-brother Josh?
 * Why was Amy's father never mentioned for a long time in the show?

Other characters

 * What happened to Leonard's ex-girlfriend Stephanie Barnett? No one knows who caused the breakup and why. and what had happened to her afterwards after her last appearance in "The Vartabedian Conundrum".
 * Is Leonard's sister's spouse a man or a woman?
 * What happened to Sheldon's assistant Alex Jensen? It's unclear what has become of her after her last appearance in "The Tangible Affection Proof".
 * Did she get fired from being Sheldon's assistant?
 * Did she resign from the job because she had had enough of Sheldon's crap?
 * When did Theodore leave?

Events

 * When did Leonard's parents get married, when did they file for divorce and when did their divorce become final?