A word to describe knowing something completely

Question

We just can not think of a word to describe this situation where you understand something completely. There is nothing you do not know about it.

If anyone knows or has something close then that would be great.

Answer 1

Comprehend and fathom are fine candidates, I'll just throw in attain enlightenment for good measure.

Edit: oh, and how about grok (Merriam-Webster, Wiktionary)?

Answer 2

fathom, though more often used in the negative (he could hardly fathom...), means to “comprehend after much thought”. comprehend also has this same connotation.

Expressions such as puzzle out or get to the bottom of have the sense of understanding completely, but also mean it was figured out (i.e. that knowledge is recent). It may or may not be adequate for your use case.

Answer 3

According to Wiktionary, to comprehend is "to understand or grasp fully and thoroughly".

Answer 4

I would use omniscient.

The story is told by an omniscient narrator.

From the NOAD:

omniscient /ɑmˈnɪʃənt/
adjective
knowing everything

Answer 5

Speaking as someone who has worked for an educational testing company, the students who demonstrated the best understanding of an essay topic were described as having a thorough understanding.

Answer 6

Informally, the term guru might apply here.

Struggling with his spreadsheet, Bob knew where to turn: Alice was the Excel guru in the office.

Answer 7

I'm not sure that the situation ever arises. The (world's) leading authority on a subject is perhaps a safer concept, but who decides who that is?

I couldn't say for sure.