Isn't this like if a tree falls and no one is there to hear it does it make a sound?
I think things/people exist, regardless of the fact that they or it can or cannot be seen or whether or not it is not know by others. An existence doesn't depend on being seen or sensed in any way it depends on a thing or persons perception of life and reactions to the things around it..

We learn this, as babies. Infants do not understand that things they cannot see, hear or touch, may still exist. This is why babies sometimes cry when their mother leaves the room â?? they think that mother is "gone," for good. When mother reappears to calm them, they're happy again. It takes some time for infants to absorb the fact that things may still exist, even if they can't be sensed.

we can be absolutely certain:
That we exist; otherwise, there wouldn't be anyone to ask do we exist...
and we have experiences, that confirm our existents no matter how small we are in the universe I think we matter and life is what you make it...

or did I miss the whole point of the question?

space cat