"In the darkness" here indicates the Fajr and `Isha' prayers which are performed when it is dark. Because of the street lighting nowadays we do not feel darkness in these hours in towns and cities but even then these lights do not overcome the darkness completely. Thus in spite of the lights, Fajr and `Isha' prayers are the prayers of the dark. Their merit mentioned in this Hadith is that those Muslims who go to the mosque for these prayers will be endowed with perfect light on the Day of Requital by means of which they will easily cross As-Sirat, while those who will be deprived of this light will be facing great trouble in crossing it.