What is the bell curve controversy?
I assume that you are asking about the controversy over the book The Bell Curve that was published in 1994 by Richard Herrnstein and Charles Murray. This book argued that there are differences in intellectual ability between social and racial groups. This book claimed that IQ tests showed clear differences in intelligence between members of various racial groups.
This led to a huge amount of controversy because of the fact that it seemed to be suggesting that non-whites are generally less intelligent than whites. This seemed like a return to the sorts of thinking that promoted things like segregation and slavery. The controversy, then, centered on the accuracy and meaning of the authors' findings. The controversy was about whether there really are differences in intelligence between various races.