Sylvia Nasar wrote the biography of John Nash, A Beautiful Mind.
It does not mention the movie's supposed dialogue about an infinite universe, but it does discuss John Nash's other views on the universe, among them
They were on their way back from Uppsala where Nash had given a talk — his first in three decades. Nash hadn’t been asked to give the customary hour-long Nobel lecture in Stockholm. The lecture at the University of Uppsala was arranged by Christer Kiselman. Nash’s chosen topic was a problem that had interested him before his illness and that he had taken up again since his remission: developing a mathematically correct theory of a non expanding universe that is consistent with known physical observations. The conventional view, of course, is that the universe is expanding, and attempting to overturn the consensus is exactly the kind of contrarian intellectual bet that Nash has always enjoyed.
but nothing about an infinite universe. In the book's Foreword, the author relates
Actually, Alicia was extremely protective of Nash’s privacy and incredibly discreet. There was only one exception: we were in the basement of her bank, sifting through the contents of her safety deposit box looking for photos. She came across these little 2 x 2 snapshots of her and John with Felix Browder at the UC Berkeley swimming pool.
and later on in the Foreward
Nash never did agree to give me an interview for the book.
It is very hard to prove a negative, but the "infinite universe" dialog is supposed to have taken place when John and Alicia were courting, which is a chapter of the book, and if such an intimate dialogue had been revealed by Alicia, it is unlikely to have been (from the above remarks) unless Alicia wanted it be included. But it wasn't.
In conclusion, there is no evidence this conversation took place, or that it represented Nash's views. The movie is a fictionalised dramatisation of the life of John Nash.