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

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

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.