Explanation:

- The idiom "dark horse" commonly refers to someone who emerges unexpectedly to prominence, often having hidden talents or abilities.

- Option 1: An expected outcome - This doesn't fit as it implies predictability, unlike the surprising element of a "dark horse."

- Option 2: An unknown genius - This aligns well as it represents hidden talent or unexpected skill, surprising to many.

- Option 3: A well-kept secret - While somewhat related, a dark horse focuses more on unexpected success rather than secrecy.

- Option 4: A horse of a fine breed - Irrelevant, as the idiom isn’t about literal horses or breeds.