How deep web are not found by search engine?

We have already talk about deep web. As we know it is hidden web that search engine can't found it or index it. The question is about why a search engine can't find these sites or data? Search engine has more power that it can show result if we search with any words but why it can't index the deep web. What is the reason?  Here I am going to write about it. First, let's know about how does search engine works, indexing process in it and many more related about it.

How does search engine works?

Search engine need to crawl the sites in perfect way so if someone search with some input keyword , it should show the best result about user input. This type of crawl technique is called bots or spiders of the search engine. Different search engine has different technique to give result as user input. In this time, google, bing, yahoo, Duck Duck Go etc are the example of search engine. Mostly search engine search the result with user targeted keyword, content, freshness of sites page, user engagement etc. While a developer do coding, they will target some keyword on the site. If the target keyword match with user keyword, it will be the best result shown on top rank on search engine. However, targeted keyword is not just a factor for top ranking. Website should have fresh content, good quality content, and sites has good organic traffic also. These are just a surface knowledge to understand search engine ranking system. Now let's go to our main point. 

How deep web are not found by search engine?

There are several ways or reasons that the deep web are not found by search engine or the sites/site's data are hidden.

• Maybe the developer/owner don't want to index their website/site's data on search engine. So they use some code on meta tag for not to index on search engine. It can be done through a line of code i.e <meta name="robots" content="noindex" /> . For not to index on specified search engine, it is also possible. You can search it for specified search engine. For google, you can use:- <meta name="google-bots" content="noindex"> . People can use no index of page/website/website's data in this way. So search engine will not index the website and it will be hidden always and search engine will  not found these data while search.
• If you use blogspot for bloging, you can see the search engine visible option. Maybe many of people do make their blog/website invisible to website. Like that there are many possibility  of many website has made this so their websites are not hidden. So these types of website will be in deep web.

• A man make a website and if he didn't register the sites with domain name. Then it will be neither index by search engine nor can show  the sites. So these types of website will be in deep web. For example, government secret data site which can't be access by any unauthorized people.
• The sites which required registration and login is also in deep web. It is not visible to all. Some people with login ID and password can access the data i.e only authorize person can see data. 
• Sometimes, some page are not linked with any pages in a website so these unlinked pages are not crawled by search engine or can't crawled. So that type of pages will be listed in deep web i.e they will be hidden.
• Let's take a example, a website is created and registered in domain name. It was shown in search engine. But after sometime, owner didn't leave the website. I mean didn't pay for domain name, and didn't buy another domain also for a long time. These type of website which is outdated are can be found in website archives. But search engine don't index these sites. So these data/sites are also listed in deep web.
• A website in which textual contents are encoded into non html format ,like multimedia files, can't be handle by search engine. So it will not index by search engine.
• Some website are hidden for regular browser but actually that can be accessed by some standard software like torBrowser, I2P etc. So these web are also listed in deep web.

This way, we have seen that how much websites/data are hidden behind the regular browser. These are just a surface view but, in actual many more websites, data are hidden. So it is true the deep web is about 96% of total web.