What does the phrase "have no idea" mean?

Have no idea is a phrase appears many times in the book We have no idea: A guide to the unknown universe, for example in the title, or in this sentence:

It's the biggest chunk of reality, and we almost have no idea what it is

My question is: What does the phrase "have no idea" mean? Can we understand it literally that people haven't got any idea about something or it means that people don't know about something?

I know that it's quite confusing and may be trivial question, but I'm translate from English to my native language, so I have to find the most exact words to describe that phrase, so please help me.

Thanks.

It's not strictly literal, the subject may in fact have some ideas. To "have no idea" means that whatever ideas the subject does have, if any, are either highly uncertain (usually, when the speaker is the subject) or presumed to be incorrect (usually, when the subject is someone else).

If I say "I/we have no idea", I mean that I am / we are highly unsure, though I probably have at least some vague thoughts on the matter. If I say "he has no idea", I mean that I think he is either ignorant or wrong.