Excerpts not showing on main page

This should output the text at the bottom of each ‘single’ post

add_filter( 'the_content', 'my_the_content_filter', 0 );
function my_the_content_filter( $content ) {
  if ( is_single() )
      global $post;      
      $content .= "<p>here will be some data retrieved from the database, it will be beneath every post. I like it!</p>";    
  return $content;

you could use is_front_page() or is_home() if you wanted the content on the home page.