10. We sent others before you, to the former communities.
11. But no messenger came to them, but they ridiculed him.
12. Thus We slip it into the hearts of the guilty.
13. They do not believe in it, though the ways of the ancients have passed away.
14. Even if We opened for them a gateway into the sky, and they began to ascend through it.
15. They would still say, “Our eyes are hallucinating; in fact, we are people bewitched.”
16. We placed constellations in the sky, and made them beautiful to the beholders.
17. And We guarded them from every outcast devil.
18. Except one who steals a hearing, and is followed by a visible projectile.
19. We spread the earth, and placed stabilizers in it, and in it We grew all things in proper measure.
20. And in it We created livelihoods for you, and for those for whom you are not the providers.
Gabriel is speaking to Muhammad about the former messengers of the other ancient civilizations. The other messengers before Muhammad were ridiculed and mocked. The disbelievers did not believe in the messages given to them because they could not see or understand the spiritual nature of things.
Muhammad is being told that even if they were to open up the gateways to heaven right before their very eyes, they would even be skeptical. So denial is placed further in their hearts as they make their choices to not open themselves to hearing the messages given to them.
Allah is explaining how the stars in the skies are clusters of constellations and they bring light and great beauty to the sky. Also human beings are protected from outside forces that wish to do us harm, except those that are stealthy enough to steal a listen to mankind, and in other Quran translations it is just “steals” so this could also mean “abduct” or “kidnap”. But these intruders are followed by a projectile missile that is set to protect humans. This can be an explanation of a galactic war or simply a more natural missile, like asteroid belts.
I do not believe we will ever fully understand those lines completely, but the main idea is to know that humans are being protected as much as possible and as much protection as allowed to us by Allah, from outside or otherworldly evil.
The last few lines explain how, Allah spread all over the world in a nature, certain laws and self sustaining systems to preserve life and keep it going until the time decided when all life cycles come to a halt.