Google thinks that www and no www as different pages and assigns different page rank for each of these. Google calculates page rank based no of backlinks you have, if you have backlinks pointing to both http://domain.com and http://www.domain.com then you will get different page ranks. If you can redirect all the back links to domain with www or without www that will help to increase your page rank.
We can acheive this by adding some rules in .htaccess file. .htaccess file is located in your site root folder, download this file and add the below simple rule to point all links to with www url. Change the domain.com in the following code to your site url. Make sure you have backup of the .htaccess file, you can restore if you mess up in the process.
Options +FollowSymLinks
RewriteEngine on
RewriteCond %{HTTP_HOST} ^domain\.com
RewriteRule ^(.*)$ http://www.domain.com/$1 [R=permanent,L]
