* Volcanic Origin: Lunar maria formed billions of years ago from massive volcanic eruptions.

* Basaltic Lava: The lava that flowed from these eruptions was primarily basaltic, similar to some types of volcanic rock found on Earth.

* Darker Color: Basaltic lava tends to be darker in color than the surrounding lunar highlands, which are made up of lighter-colored anorthosite rock.

* Low Reflectivity: The smooth, glassy surface of the solidified lava reflects less light than the more rugged, cratered highlands, contributing to their darker appearance.

So, the dark color of lunar maria is a consequence of their volcanic origins and the composition of the lava that formed them.