Did you know that when you perform a web search you're not actually searching the internet in real time? Your results are just what the search engine "knows about." Sometimes the website creator submits the web address directly to the engine, but usually the search engine knows about a website thanks to its web crawler. 

All general-purpose search engines have a web crawler that visits pages on the internet, automatically following one link to another. After the crawler visits the page, it submits the text on that page to an indexing program, which stores the text in a database. The resulting database is what you're actually searching. That means that if for some reason a crawler hasn't visited or indexed a particular page, that page will not show up in any of the search engine's results.

Source. 

  How Searching Works