what are grey patches on moon which we see from earth?

Answer 1

When you look at the Moon, you will see dark and light areas. The dark areas are young plains called maria and are composed of basalt. The basalt flowed in and flooded the area created by a huge impact with an asteroid or comet. The light areas are the highlands, which are mountains that were uplifted as a result of impacts.

Answer 2

The dark patches we see are basically evidence of impacts in the moon's past. As asteroids struck the moon, underlying lava was pushed to the surface where it cooled, leaving the dark seas we see from earth.