What is the Quran referring to when it mentions, "He has created the skies without the pillars"?

According to Quran 31:10 : > He has created the skies without the pillars that you may see, and > placed mountains on the earth, lest it shakes with you, and spread > over it all kinds of creatures. And We sent down water from the > heavens, and caused to grow in it every noble pair. The sky is just the color we see. It is not a structure, or a roof over our heads. So what is the Quran referring to when it mentions, "He has created the skies without the pillars"?