The picture is not darker. The parts on the screen that are supposed to be deep black are actually deep black.
With LCD/LED backlit TV's, you have lamps arranged behind the screen, and when something on the screen needs to be bright, the lamps brighten. Unless you have a TV with "local dimming" technology (lots of small lamps arranged in a grid behind the screen) there's lamps arranged behind the screen long ways. When one of the lamps lights up, the light 'bleeds' into other parts of the image that shouldn't be lit, thus making sections that should be pitch black look dark grey. In dark scenes, you can see the back lighting.
Each pixel of the 2 million or so on a 1080p screen light up individually on Plasma TV's, so pixels in parts of the image that are supposed to be dark simply do not light up giving you true blacks.