system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is the Multiverse and why do we think it exists?

I saw it on Cosmos, and don't understand the idea that there are many "bubbles" that each contain a universe. How does that reconcile with the Big Bang theory where space expands from one single point? How would a multiverse start?
gpt
In mathematics and physics, you can ask questions with mathematical equations or series of equations and proofs.  Generally, without getting too far into it, when quantum physicists specifically address this question, the math sometimes suggests that multiple universe exist.  

It takes some imagining if you don't understand such advanced mathematics.  But generally speaking, equations tell us something about our universe, even simple ones like 1+1=2.  These very clever and thorough physicists are trying to tell something bigger than that.  And sometimes the math tells us that multiple universes must exist using the logic and math we understand.