If you goto your Settings in the dropdown under your Name, you will see a Checkbox under Message Preferences that allows "Open Site Links in a New Tab." That way when you click on a story, you still have your original place in the Gulch.

The only time this should occur is if someone is linking to an outside story.

For example, if I start a discussion and in the URL field link to The Atlas Society, when people click the title of my post, they will be taken to wherever I linked to.

Does that make sense?

You can tell you're about to go to a different site by looking at the URL in parenthesis after the post title. For example, right now there is a new post by nickursis entitled "San Francisco proposes a tax on robots." After the title it says, in smaller print, (yahoo), so I know if I click the title of his post I'm going to a Yahoo page.