Question

Why do astronomers think of the universe as having a structure similar to soap bubbles?

Answer 1

Well some people think that the universe is faulty, empty and unstable, just like soap bubbles.
Hope this helps!

Answer 2

Another theory to put into consideration (And mind you, it is only a theory) is that there are multiple universes that exist in their own little 'bubble', and they are constantly floating around each other. This could be an attribution to the thought of a 'soap bubble', however, the first answer given might be more accurate.