When our CEO retired last year, there were a number of senior managers in our company who really wanted the position. But in the end it went to the dark horse (unbekannter und erfolgreicher Außenseiter), a woman from outside our company who was 15 years younger than any of the other candidates and had no previous experience leading a company.
This idiom comes from the world of horse racing (just like this one). A dark horse was one that wasn’t known to the the people betting on the race, and was difficult to place odds (Wettquote) on. From here it came to mean any little-known person or thing that reaches prominence.