Dell E173FP monitor

You seem to be describing normal behavior. Some websites are designed to flow or stretch to fill the available width of your browser window. Other websites are designed to be of a certain, fixed width. If the page is wider than what your browser can display, you'll normally see a horizontal scroll bar which you can use to make the hidden content visible. If the page fits perfectly within your browser window, you'll see no horizontal scroll bar and no blank areas. If the page is narrower than what your browser can display, there will be a blank area somewhere (on the left, on the right, or on both sides).

Fixed-width layouts are popular. Since alot of people are still running at 800x600 screen resolution, and sites want to reach the largest audience possible, they choose a layout width than can be displayed within a maximized browser running on a 800x600 res system. However, most people are actually running at 1024x768 or greater, so those sites won't fill a wide browser window on their system... they'll see the blank areas you seem to be describing.