With this sentence they're teaching us that "todo el mundo" means "everyone" - if they accepted "the whole world" that would be ruined, in my opinion. We're here to learn a new language, not to get every answer correct on the first try. Accept and appreciate that other languages express themselves differently to English!
"The whole world" is also idiomatic in English, a mild hyperbole on "everybody." No reason for "The whole world is here" to be marked wrong. We express ourselves the same way in English, French, and Spanish. C'est vrai! BTW, "todos" can be used in Spanish to mean everyone. But I will answer "everybody" next time so that the green owl doesn't mark me wrong. Is that learning or a conditioned response?
Kaboom! Good streak going. Answering to questions happily like always and there it comes out of the blue. Idiom wolf, looking like normal harmless literal sentence sheep, but ready to bite your streak down with its sharp symbolic teeth.
4.5.2020 and it still gives the wrong answer for "The whole world is here". There would be two easy options to solve this types of things:
Moving this type of idioms to idioms-category.
Accepting the literal translation as correct and adding an explanation to the bottom of the page about the meaning of what the idiom symbolizes. In many languages there is similar (word to word) idioms, but not in every language.
It wouldn't take a lot to fix these things. There is not really a lot to complain about Duolingo, especially since it can be used as a free program and its no doubt one of the best in its class. Duolingo is very smooth and user friendly, but guiri like me needs some small things to highlight and rebel the mighty power of Duo-owl.