web site doesn't respond to www.example.com address

i have a domain, say example.com and I host web site using the Apache web server

my website config file (/etc/apache2/sites-available/example.com) looks as follows:

 <virtualhost example.com:80="">ServerName example.com
    #ServerAlias www.example.com
    DocumentRoot "/var/www/sites/example.com/current/public"
    ErrorLog "/var/log/apache2/example.com-error_log"
    CustomLog "/var/log/apache2/example.com-access_log" common
    <directory "="" var="" www="" sites="" example.com="" current="" public"="">Options All
        AllowOverride All
        Order allow,deny
        Allow from all</directory> 
    RewriteEngine On
    # Remove the www
    RewriteCond %{HTTP_HOST} ^www.example.com$ [NC]
    RewriteRule ^(.*)$ http://example.com/$1 [L,R=301]</virtualhost> 

The website responds to example.com address but doesn't respond to ****www.****example.com

Am I missing something?