Do plants grow faster in dark or sun?

Plants require light for photosynthesis, so it is impossible for plants to increase in dry biomass without any light.

Some plants do not require high levels of light for growth, because they are adapted to low light conditions (such as jungle and forest plants).

Plants left in the dark will grow taller than plants in lighted conditions over a short period of time, but they will also be yellower and more spindly (etiolation). The increased height is a response to low light levels - the plant is trying to grow over anything that may be blocking the light, and will grow towards any source of light.

As mentioned in the discussion page of this question this response is controlled by auxins in the plant growth points.

For very short periods of time the plant will appear to be going faster than a control plant in normal light intensities - however, this is because the stem is normally thinner so the amount of biomass produced is roughly the same. In addition the plant has to make use of stored energy reserves as it is not able to photosynthesis.

Etiolation is a form of self-protection mechanism, the plant expends energy and reserves looking for a source of light that can ensure its continued survival.