Indoor picture are dark when using fla...

If you are in Manual Mode and you set your shutter speed too higher (ie above 250th sec) the shutter blind passes over the sensor when the flash goes off causing a dark section in the photo. Lower the shutter speed to get a better sync. Some camera's default sync speed is 1/60th of a second and sometimes user adjustable.

Remember the longer the shutter speed, the more light will register in the photo. Also boosting the ISO will help extend the flashes range and also record more ambient light for a more pleasing and balanced photo.