It is a WEB search engine?

Does Elasticsearch do research on the web?
I post this question because all demos that i have seen don't treat the web serach !

You can build it to do on site search. Lots of folks do. But it doesn't contain any crawling or similar features - just the storage of the page and search etc. Its a building block.

In fact, i'm working on a question answering system. What i need is a web search engine that takes as input the question keywords, and returns a set of ranked passages that may contain the answer.
Is Elasticsearch able to do this?
if not, please can you sugguest me other web search engines?

Sure. That is exactly what it does.

Okey. Please, can you send me a demo that illustrates this ?
(i did not find a demo concerning the serach on the web)

Wikipedia does that.