There are a number of criteria by which websites are judged by search engines, during the process of ranking search results. One of these criteria is popularity. If search engines believe your site to be a popular and valuable source of information with regards to a particular search topic, they will feature you highly within their results list.
Take a look at the links to your homepage throughout your website. Does each of these links include the appendage index.html, such as http://domain.com/index.html ?
If so, you are splitting your links. This is bad for Search Engine Optimisation (SEO) and that is bad for you.
When the homepage links on your website are appended with index.html it means that you’re splitting these links, and subsequent traffic from them, apart from the external links to your site. External links without the suffix index.html will go directly to your domain name, http://domain.com. Splitting your links means that you’re not doing your SEO practices any justice.
As already stated, one of the ways to earn a higher ranking within search engines is to cultivate popular links. The more popular and trusted a link, the higher it is likely to appear in relevant user searches. The problem with splitting links is that you’re dividing that popularity between two separate destinations, thus unlikely to reap the rewards that your SEO practices deserve.
Remove index.html, default.php or any other appendages from all of the links to homepage across you website, and instead link to your domain. That way all external and internal links will be alike and working toward the same endgame, which is propelling you up the search listings.