Why can the structure of the universe be compared to soap bubbles?
Excellent question! On the very largest scales we observe that clusters and super-clusters of galaxies exist around voids.
Seen from afar, the distribution of galaxies isn't random as we might suppose. It appears to be in a web, much like a spider's in 2D, or in bubbles in 3D. This fits well with predictions by the leading theories in cosmology, such as LCDM.
This video makes for an interesting 5 minutes that should provoke more questions.