How to make the content fills the space after the sidebar ends in a drupal site?
This really boils down to what theme your using, and how your content is laid out. But it should be doable with CSS if your theme is laid out correctly. Let's take Bartik for example (one of Drupal's default themes). Simply changing the sidebar-first class to float left and changing the float of the content to none will give you something like this: 



And here's a really basic version the same idea running on JSfiddle.

The basic answer of it is, yes, it can be done, but you need to do it through the theme and css. Depending on where blocks are placed, the css already in place on your theme, how you go about doing it might change, but the answer is you need to use css.