Lightning occurs as a result of charge separation in a cloud. When negatively charged electrons build up at the base of a thundercloud, anything it passes over will become positively charged. If the cloud passes over a tall object, like a skyscraper, the electrons will make the jump, creating the pitchfork of light you see streaking across the sky.
Scientists have realized though that the process of creating regular lightning produces sprays of x-rays and gamma rays that are also ejected into the atmosphere. This is what scientists are now calling dark lightning. Though we can't see dark lightning, it produces some very measurable cascade effects.
To hear more about dark lightning, view NASA's video about it below: