The Dark Knight has become only the second film in history to top $500 million in U.S. movie ticket sales.

Although U.S. movie ticket sales hit the doldrums during the past weekend, The Dark Knight reached a milestone, becoming only the second film in U.S. history to earn at least half a billion dollars in U.S. theaters.

That’s an impressive performance, but most analysts don’t think the film will catch up to all-time leader Titanic, which earned $600.8 million in U.S. theaters. Most are expecting Dark Knight to top out at about $530 million, though $550 million is possible if it can continue at a stronger than expected pace.

In addition, because ticket prices are significantly higher today, Dark Knight would have to breach the $900 million mark in order to sell the same amount of tickets as Titanic. Obviously that’s very unlikely.

Still, it’s a hugely impressive and somewhat unexpected audience reaction, especially for an action genre film that could be expected to have less than ideal appeal for female audiences. Clearly, co-writer and director Christopher Nolan deserves much credit for bringing sufficient intelligence and emotional maturity to the story, and the performers, particularly the late Heath Ledger, found ways to make the potentially cartoonish characters recognizably human.

Even so, there must be something special in the ideas of a film for it to have such strong appeal for audiences, some way that it has captured the zeitgeist. As noted earlier, I suspect that what captivates audieces is the film’s mature and sophisticated recognition of the reality of moral choices. For a society of people who have been taught moral relativism in the schools and throughout much of the culture but recognizes from life experience that some choices people make do indeed harm themselves and others, the film’s stout declaration that morality is real and powerful is a bracing and inspiring thing indeed.

In that regard, The Dark Knight has earned every nickel it has made.