No one has ever seen a particle or a wave of gravity. Furthermore, the very notion of gravity is mystifying: masses light years apart with nothing between them, affecting each other’s movement!

Yet we all accept there must be a cause behind the phenomena we observe, so we call this elusive force “gravity.”

So too, there is a cause behind existence. That Cause may be yet more elusive, but the reality of its existence is at least as inescapable and empirically evident.

-from correspondence of the Rebbe to Zalman Shazar.