expression or idiom for a person who knows every detail about something

I was wondering if there's an idiom or expression for a person who knows everything there is to know about his field of interest or expertise, practically no detail is unknown to them.

I thought of the expression: knowing like the back of hand, but I guess that has a geographical sense to it and is used for cities, streets and neighbourhoods. (Please correct me if I'm mistaken.)

I hope I was clear. Thanks in advance.

In this situation, I would say he "knows it backwards". The implication is that one knows something so thoroughly that they could do it in reverse.

Also consider

"knows it backwards and forwards"

{Cambridge Dictionary of American Idioms}

and "knows it inside out"

{McGraw-Hill Dictionary of American Idioms and Phrasal Verbs}.

"knows it like the back of his hand" does often, but not always, have a geographical connotation.

One idiom that people in the United States sometimes use to describe a person who possesses extensive, detailed knowledge of a particular subject is walking encyclopedia. Here is the entry for that term in Christine Amer, The American Heritage Dictionary of Idioms (1997):

walking encyclopedia A very knowledgeable person, as in Ask Ron—he's a walking encyclopedia of military history. A similar expression, a walking dictionary, was used by George Chapman in his poem "Tears of Peace" (c. 1600)