About 5 billion years ago the sun formed out of part of the junk resulting from a supernova. The sun condensed, but around it there was a cloud of interstellar dust and hydrogen gas. As the sun got hotter, the cloud around it cooled and began to separate into chunks of various elements, some of which were large enough to be planetesimals. Over a very long period of time, collision with smaller chunks created larger and larger planetesimals. Finally, the sun ignited, which blew away much of the gas around what was becoming our planets. Finally, a collision with something pretty big knocked a chunk of the earth out into orbit, where it eventually became the moon. For a longer and perhaps clearer explanation, see link below.
Tuijor’s explanation is very good — I just want to emphasize the importance of gravity. Inevitable concentrations of mass didn’t collide so much as combine towards a center of mass, creating stars when the matter was hot, and planets, asteroids, comets, etc. when the matter was cool.
see also at least 5 previous answers
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