How did the lunar craters and mares originate?

The moon is covered with craters. There are about 30,000 craters on the side of the moon that faces us. They originated about 3.5-4 billion years ago, when chunks of matter moving around in outer space hit the surface of the moon. As there is no atmosphere or wind on the moon, these craters created by meteorites were not blown away and are preserved till date. The large dark spots on the surface of the moon were earlier believed to be seas and were named 'mares', which is the Latin word far 'seas'. These basins were probably impact craters of asteroids, which were filled with lava. The bright areas on the moon are known as 'terra', which means 'mainland'.