eli5 What do we know about what’s outside the observable universe?
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Absolutely nothing. The definition of “the observable universe” is everything we are capable of knowing about.
Unless we invent faster than light travel it is forbidden by the laws of physics to ever know anything about what is outside the observable universe.
The reason why we assume there is anything out there is because all of our theories of physics expect that we live in a small corner of a bigger universe. Additionally, the observable universe grows over time and we haven’t seen anything resembling an edge.
The reason why an “observable universe” exists at all is that it takes time for light to travel. The observable universe is simply everything where the light has had time to reach us.
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