Only beings who have consciousness are able to experience existence or have knowledge about it. Quality of knowledge depends of inteligence. Different living creature have different inteligence therefore they have different experience or knowledge.
Human have the most complex inteligence on planet Earth what cause them to have the most complex experience or knowledge about existence. We still don't know are there somewhere in universe beings with more complex inteligence and knowledge then us. If we have the most complex inteligence on planet Earth, it dosn't mean that is the most complex in whole universe.
Everything points on inferiority of human inteligence not on superiority. Physical existence by itself represents one complex system of different consciousness and they interaction. Human inteligence represents only part of that system with specific function and purpose. We still don't understand complexity and function of that whole system what again points on our inferiority and not on our superiority in frame of that system.
We determinate existence by analysis of physical existence. Base of that analysis is capability of human inteligence to notice and to link everything noticed. Determination of existence depends of some factors as:
If is something present in physical existence people can notice it. If is something absent from physical existence people can't notice it. That absence people experience like non existence.
Also, sometime even if is something present in physical existence, people are not capable to notice it. For long time people wasn't aware of bacterium.
Therefore, existence for people is always determinate by them consciousness. Consciousness by itself represent one constant activity and process what cause that we never have absolute knowledge about existence, but always relative.