LOS ANGELES (Reuters) - New Batman movie "The Dark Knight" debuted in North America on Friday to sold-out theaters and a record $18.5 million in screenings that started one minute after midnight. Box office watchers are saying it could break records for weekend ticket sales. Following are six key facts about "The Dark Knight."

-- "The Dark Knight" is based on the Batman series of DC Comics, and director Christopher Nolan has drawn particular inspiration from dark and somber depictions of the Caped Crusader in a series of graphic novels about Batman.

-- Nolan also made the 2005 movie "Batman Begins," the previous film in his series, and it grossed roughly $372 million at worldwide box offices. His breakthrough film was 2000 noir thriller "Memento."

-- In "The Dark Knight," Christian Bale stars as Batman and the late Heath Ledger as the Joker, Batman's most notorious villain. Ledger was found dead in his New York City apartment on January 22, 2008. He died of an accidental drug overdose.

-- "The Dark Knight" is the sixth installment in Warner Bros.' series of modern Batman movies, which started in 1989 with director Tim Burton's "Batman," starring Michael Keaton as the superhero. Other installments include "Batman Returns" (1992), "Batman Forever" (1995) and "Batman & Robin" (1997).

-- Recent movie versions have been violent and starkly different from the "Batman" U.S. television series starring Adam West, which offered a more camp and humorous take on the comic book hero.