Dark reaction is the process of photosynthesis, which happens in the absence of light, i.e., it happens in the night, but it is also the reaction that is happening simultaneously with light reaction. It's a chemical process that happens in a plant that does not need light. Moreover, this reaction is based on the enzymes reacting with each other. As a result, even if there is no light, it will still take place. Also, when it comes to photosynthesis, it can only be done when sunlight or artificial light is triggering photosynthesis.