That is not science

It is originated from chinglish "zhe bu ke xue". It is always used to express surprise or astonishment at unexpected or unusual things. By saying that, one means the thing he/she is told or sees is out of his/her expectation.

A: You know what, Tom just got the offer from Morgan Stanley, but he rejected it and decided to go to a small local bank.

B: Oh, that is not science.