The possible causes for a laptop screen to display darker than usual can be:

1. Your battery was low at sometime and the laptop switched to a lower light intesity to conserve energy.

Solution: Check the intensity level from your Dell utility.

2. Your backlight life cycle is near the end.

Solution: you have to take it to a laptop/pc service to examine/replace it.

3. Some laptops have a small button that when the lid closes it is pushed down.

Solution: a common issue is the button to get stuck and the laptop to 'think' that the lid is closed. Check if you have a button like this(located at the junction between the monitor and the laptop unit) if it's stuck.

4. Uninstall and reinstall your video card drivers.
5. Turn off all screen savers/hibernate/standby functions.