While Portman did extensive training to physically prepare for the role, she also worked with professional ballet dancers as doubles for some of the more difficult and technically demanding sequences.
The movie's director, Darren Aronofsky, wanted the dancing to feel as authentic as possible. He hired the renowned choreographer Benjamin Millepied (who later became Portman's husband) to create the ballet sequences and train Portman.
So, while Portman herself did a lot of the dancing, there were also other dancers involved, particularly for scenes that required specific technical skills. But Portman's performance, which included her own impressive physical transformation, is what truly made the movie's dancing so compelling.