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Warner Bros.' Batman sequel "The Dark Knight" earned a record-breaking $75.6 million in its second weekend from 4,366 theaters, bringing its domestic cume to $314.2 million and putting it on track to reach $400 million in slightly more than a week from now.

It was another sunny weekend at the domestic B.O. overall. Sony's Will Ferrell-John C. Reilly comedy "Step Brothers" opened to plenty of laughs in grossing an estimated $30 million from 3,094 runs, while Universal holdover "Mamma Mia" dipped only 36% in its second weekend to grab an estimated $17.9 million from 2,990 runs for a cume of $62.7 million in its first 10 days, according to Rentrak.

The only title coming in on the lower end of expectations was 20th Century Fox's "The X-Files: I Want to Believe," which opened to an estimated $10.2 million from 3,185 runs. Pic placed No. 4, behind "Dark Knight," "Step Brothers" and "Mamma Mia," respectively.

Studios weren't sure they could match the strength of the same weekend a year ago, when Fox's "The Simpsons Movie" opened to a boffo $74 million. The weekend was down only slightly from last year by roughly 1%.

Thanks to the runaway success of Christopher Nolan's "Dark Knight," Warners crossed the $1 billion mark in 2008 ticket sales on Sunday, earlier in the year than ever before.

Warners predicts it will overtake Paramount for top in marketshare by the end of the coming week.

Imax continues to see record-breaking results as well. (Nolan shot several segments with large-format cameras.) For the weekend, "Dark Knight's" Imax haul was $4.6 million from 94 theaters for a per location average of $48,500.

The previous record-holder for best second weekend at the domestic box office was "Shrek 2," which earned $72 million over Memorial Day weekend. "Dark Knight" didn't have the benefit of a holiday either.