No one has ever seen, touched, or measured a gravitational particle or wave. Moreover, the very notion of gravity is mysterious: masses light-years apart with nothing between them, affecting each other's movements.
Even so, we all accept that there must be a cause behind the phenomenon we observe, so we call this elusive force "gravity".
Similarly, there is a cause behind existence. That Cause may be even more elusive, but the reality of its existence is at least as undeniable and empirically evident.