What is the origin of the idiom "gray matter" and when to use it?

The idiom "gray matter" originated from the color of the human brain, which is often described as grayish in appearance. It has been in use since the 19th century, and the term "gray matter" has long been associated with intelligence, intellect, and the cognitive abilities of the brain. It is often used in a positive or neutral context to describe someone's intelligence or ability to think critically.