Known Facts:
* Early Life: Harry discovers that Dumbledore was raised in poverty, lost his mother and sister, and had a complicated relationship with his father. He learned about Dumbledore's sister, Ariana, and her tragic death.
* Youth and Grindelwald: Harry learns about Dumbledore's youth and his close friendship with Gellert Grindelwald, their shared ambition for a "better world," and their eventual tragic falling out.
* Time as Headmaster: Harry learns about Dumbledore's time as Headmaster of Hogwarts, his dedication to protecting the school and the wizarding world, and his efforts to fight against Voldemort's rise.
* Secret Love: Harry learns about Dumbledore's unrequited love for Gellert Grindelwald, and later discovers a secret love for his old friend, the half-giant, Rubeus Hagrid.
* The "Secret" of the Peverell Brothers: Harry learns about the Peverell brothers, the origin of the Deathly Hallows, and Dumbledore's connection to them through his mother's side.
Potential Areas for Further Exploration:
* Dumbledore's Early Magic: We know Dumbledore was a brilliant wizard from a young age, but we don't see much of his early magic use. There could be stories about how he developed his powers, the first spells he learned, or how he discovered his talent for transfiguration.
* His Time as a Teacher: While we see a glimpse of Dumbledore's teaching style, there could be more to discover about his time as Transfiguration Professor. He may have had memorable lessons, interactions with specific students, or even a personal connection to certain subjects.
* More about Grindelwald: We see some of Dumbledore's past with Grindelwald, but there could be more to learn about their relationship. What happened to Grindelwald's plans after the duel with Dumbledore? What were the specifics of their shared ambition for a "better world?" Did Grindelwald ever truly regret his actions?
* Dumbledore's Other Allies: We know Dumbledore had allies beyond Harry and the Order of the Phoenix, but we don't see much of their interactions. Did he have any other mentors or friends who shaped his life?
* Dumbledore's Mistakes: Despite his brilliance and good intentions, Dumbledore was flawed. He made mistakes, some of which had lasting consequences. Harry could have explored these mistakes, their impact, and Dumbledore's own regrets.
Overall:
While there are no confirmed "secrets" about Dumbledore's past that Harry discovers in the books, there is much potential for exploration and interpretation. The details presented provide a rich tapestry of his life, and readers can imagine their own explanations for these moments.