If it is invisible to your eyes or your other senses,
We probably need to expand that to 'invisible in principle'. I have never been to Kashmir and have never experienced it, but I wouldn't want to deny its existence. Others have been there and I could go there too, if I wanted.
The words 'in principle' probably need stretching. I assume that there are countless exoplanets in the Andromeda galaxy, but absent the invention of super-luminal travel, I don't expect that any human being will ever visit any of them. Doesn't mean that they don't exist though.
And we would probably have to include instrumental extensions of our senses. Telescopes, microscopes, radio receivers and all kinds of things reveal realities not immediately apparent to our senses.
why is it impossible that it does exist without you being able to detect it?
I would never want to make that claim.
There doesn't seem (at least at first consideration) to be a plausible chain of inference leading from 'there's no way that I (or anyone else) can know about some unknown X' to ' there's no way that the unknown X can possibily exist'. That looks like a
non-sequitur to me, unless we introduce some unstated idealistic premise along the lines of 'to be is to be perceived'.
Can it be that other people can see and understand something you can't see or understand?
Sure. The Kashmir example addresses part of that. I've never been to Kashmir and have never experienced it for myself. But I trust that others have been there and I trust their testimony. My personal experience doesn't seem to be necessary for something to exist.
And there are physical things that I don't really understand, but that physicists not only believe are real but also believe that they have instrumental evidence of that reality (after a whole lot of theory is applied). My understanding something doesn't appear to be the criterion of whether it exists or not.
So bottom line, I'm an
ontological realist. I hold that countless things (objective things as opposed to subjective things) have their own mind-independent reality. They are
discovered, not created when sentient beings become aware of them. Their reality depends in no way on us.
If things exist in their own right and aren't dependent for their existence on me, and if its entirely possible for things to exist that I know nothing about and couldn't understand if I did, I think that I want to say that the reality that I know sort of fades off into the fog of the unknown at the edges. I'm quite certain that reality is much bigger than what I know (and probably what any human can possibly know). The boundaries of the unknown are unknown by definition. So I can't really say with any certainty what exists and doesn't exist beyond the very narrow scope of my awareness. That's why I'm an
agnostic regarding transcendental matters I guess.