Actually Newton didn't have a clue as to the inverse square law and we still don't.
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Fair enough.
That being the case, the only honest answer to a pestering (I mean, curious) young Ravi's question would be to state the theory, then say flatly that the full answer continues to elude us. Even so, it seems to be very useful as an explanatory tool and reliably describes various observable phenomena (insert interesting examples here).
It will suffice till something better comes along.
Perhaps it's your job, young Jedi, to provide that better answer.
.d.