Developers unveiled the design Thursday for the performing arts center planned as one of the final facilities to rise at the World Trade Center site.

The cube-like, 90,000-square-foot center is to glow from within thanks to translucent marble walls, and Barbra Streisand will serve as chairwoman of its board.

Simultaneously, on the other side of Ground Zero, the National September 11 Memorial & Museum placed on display the American flag that was raised by three firefighters over the rubble in the hours after the 2001 terror attacks — a symbol of hope and resilience captured in an iconic photo.

The events come as the city and nation prepare to mark the 15th anniversary of the horror that took nearly 3,000 lives and set off a determined — and politically complex — journey to recover and rebuild in lower Manhattan.

The Ronald O. Perelman Center for the Performing Arts at the World Trade Center will stand in contradiction to the hate behind the attacks, the namesake billionaire benefactor said.

“We are presenting to the world, as the United States and as New York City, an alternative to hate and destruction and terrorism and that is the opening of arts and the opening of our minds and the opening of our hearts,” Perelman of Manhattan said.

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Architect Joshua Prince-Ramus said the structure would include three auditoriums and a rehearsal room with “guillotine walls” that could be moved to create 11 different configurations.

The $250 million facility is slated to open in 2020. Perelman donated $75 million and the state-city agency Lower Manhattan Development Corp. has committed $100 million in federal funds.

It will a home for dance and music, a host of the Tribeca Film Festival and a “birthplace” for the creation of art, center president Maggie Boepple said.

The 3-foot-by-5-foot American flag that is now part of the Sept. 11 museum had disappeared shortly after the shot of the firefighters was taken by photojournalist Thomas E. Franklin, of the Bergen Record in New Jersey, and returned anonymously to police in Everett, Washington, museum officials said. Its authenticity was verified after months of forensic testing, officials said.

“The raising of this American flag by our first responders helped reaffirm that the nation would endure, would recover and rebuild,” Sept. 11 memorial president Joe Daniels said in a statement.