Here's why:

* The reaction is photochemical: The reaction between methane and chlorine is a photochemical reaction, meaning it requires light energy to initiate.

* Free radicals: The light energy breaks the chlorine molecule (Cl₂) into chlorine free radicals (Cl•). These free radicals are highly reactive and initiate the chain reaction with methane.

* No initiation in the dark: In the absence of light, the chlorine molecule remains intact, and there are no free radicals to start the reaction.

The overall reaction in the presence of light is:

CH₄ + Cl₂ → CH₃Cl + HCl

This reaction produces chloromethane (CH₃Cl) and hydrogen chloride (HCl).

In summary, while methane and chlorine can react, they do not react in the dark because the reaction requires light energy to generate the reactive free radicals necessary for the reaction to occur.