I think the reason some people might believe this claim is because we’re taught in school that the moon’s gravity causes the tides. I think the reasoning goes, “well if the moon’s gravity can affect the tides, surely it can affect smaller things too”
Would that logic not extend to all mass in the universe, regardless of observability?
Yes I believe so. Adding "observable’ was unnecessary on my part.
I’m not so sure. Gravity only travels at the speed of light so mass outside of the observable universe hasn’t been able to affect us yet.
This is a great point. I remember last year ligo detecting g waves from the merging black holes and experimentally verified this.