Select a slower shutter speed so that your camera has light entering the lens for longer. A shutter speed of 1/1000th of a second is incredibly fast, and will allow hardly any light through to create your photo. Light is transformed into information to produce a photo by your camera.
Use a higher ISO so that your camera is more sensitive to the available light. This comes at a price though, as the higher the ISO, the grainier your image will become.
Grain is also referred to as 'noise' in an image.
A wider aperture (a smaller number) will let more light in and brighten up your photos. Something to remember when using very wide apertures, such as 1.4 or 2.8, is to keep your eye on your focusing.
The wider the aperture = the smaller the area of focus on your subject.
So for photos that are coming out too dark use:
Slower shutter speed Wider aperture Higher ISO