What is the origin of the idiom "dark horse" and when to use it?

There are a few theories about where the phrase "dark horse" comes from. One theory is that it originated in horse racing, where a dark-colored horse that is not well-known or expected to win a race can surprise everyone and come out on top. Another theory is that the phrase originated in politics, where a little-known candidate could surprise everyone and win an election. This theory suggests that the term "dark horse" was first used in the United States in the mid-1800s in reference to James K. Polk, who was a relatively unknown candidate but ended up winning the presidential election in 1844. Regardless of its exact origin, the phrase became more widely used to describe any person or thing that is unexpected or surprising in terms of their success or abilities.