You have it backwards. Existence is the property shared by things which are or could be observed. If [an alternate universe]/[god]/[exotic laws of physics] "exi

You have it backwards. Existence is the property shared by things which are or could be observed. If [an alternate universe]/[god]/[exotic laws of physics] "exists" but can't interact with anything else around us in a measurable way, then what does it mean to exist anyway? It might as well not.