Plants do not actually grow faster in the dark; they only appear to grow faster because of rapid cell elongation. Rapid cell elongation is an emergency response through which the plant stretches its cells in an attempt to reach a light source.
While rapid cell elongation causes plants to appear longer, it is not actually growth. The process is more comparable to stretching. Plant cells mostly expand upward when exposed to total darkness, because that is generally where they can access light; this process is known as geotropisim. Green plants require photosynthesis in order to experience actual growth; plants left in total darkness for an extended period of time eventually die.