"It's a bit like searching for a needle in a haystack," said French scientist Jean-Pierre Vialle, part of the international team that worked on the AMS-02 project.
"But if we find it, it will show beyond a doubt that stars made of antimatter exist in some part of the universe. That would be a major revelation."
The STS-134 mission, initially set to begin April 29, was delayed hours before liftoff when technicians discovered a power failure in a heating line that served to prevent fuel from freezing in orbit.
But NASA completed exhaustive repairs last week. U.S. Congresswoman Gabrielle Giffords of Arizona, the wife of shuttle commander Mark Kelly, traveled to Florida with family of another astronaut and watched the launch from Kennedy Space Center, her office said.
"Gabrielle & Mark said goodbye during afternoon visit before launch," said a status update on her Facebook page on Sunday.
Giffords, who was allowed a break by her rehabilitation doctors in Houston to watch the planned April 29 launch, is recovering from a gunshot wound to the head, after she was attacked in January during a meeting with local voters.