How do I remove white space from Bartik?
First, you should not attempt to manually override and of Drupal's core files (or any other module or theme for that matter) because your changes will be overwritten during updates. So do not search for the CSS and attempt to alter it.

Make a copy of the bartik theme instead, move it to sites/all/themes and rename the folder and the .info file. Finally alter the info file to give your theme a new name. 

All Bartik lay-out code can be found in layout.css. So you will have to modify widths and margins there.

Finally, I wanted to make you aware of the Responsive Bartik theme which is already slightly wider for widescreen monitors, but also optimized for mobile phone usage. You might find it usefull to use that instead, or use it as a base theme for your modifications. 

Update for Drupal 8

You have the core/themes/bartik/css/layout.css file where you have to alter the max-width for wide screens:

/**
 * Container
 */
.layout-container {
  max-width: 860px; /* &lt;- This is the default width. */
  margin-left: auto;
  margin-right: auto;
  box-sizing: border-box;
}
@media all and (min-width: 851px) { /* &lt;- Bigger than this means it is a wide screen */
  .layout-container {
    max-width: 1290px; /* &lt;- This is the max-width for wide screens. */
  }
}


Most of the CSS properties are defined in percents and so they should scale along.