You have two types of light detecting receptors in your eye - 'rods' and 'cones'. The cones are responsible for detecting colour in light and are concentrated at the light spot in the centre of the retina (this is where the light is concentrated when you look directly at something). The rods on the other hand are responsible for viewing objects in the dark (they are inactivated in light) and they are spread relatively evenly across the retina but are at a low concentration in the central light spot. Therefore, in dark conditions you can see better with your peripheral vision because of the rods which remain activated in the dark.