In a light beam, the flow of light through space is similar to water flowing in a river. Even though the light from something like a laser pointer travels in a straight line, it can also flow in whirls and eddies. These swirls of light are called optical vortices. Along the vortices the intensity of light is zero, i.e. darkness. The light all around us is filled with these dark lines, even though we can't see them. In natural fields the lines map out fractal paths but using specially designed holograms one can twists these dark filaments within the light into knots.