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”
On a very very technical level kinda yes, a weird quirk of gravity is that it has infinite range even though it follows the inverse square law. This means that the moons gravity imparts some amount of force on you. Not just the moon either, all objects in the observable universe impart some force on you, as do you impart some force on them. The problem Is that this force is so absolutely infinitesimally small It may as well be zero. So in the world of theoretical physics yes kind of but your friend probably isn’t a physicist. Sounds like they’re more woo woo astrological magic type people that believe the moon imparts some metaphysical healing magic on their insides.
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.