How to darken the active screen using a shortcut-key in Windows 7? [duplicate]

Is there a key combination or keyboard shortcut that causes the screen to go dark while the PC computer is working?

Example: I am listening to songs with Windows Media Player or I am listening to a lengthy YouTube clip. It is night-time and I have the lights out in the room but the monitor is giving off glare. So I’d like to just press something and cause the screen to go black in an instant. Then I would need to be able to instantly revive the screen at any moment.

Does such an instant function exist?