In fact since Humans exist they know that objects fall and that the moon turn around the earth.
But it's only recenltly in history that the exact fomula has been given, that attraction is proportional to mass and the inverse of the square of the distance. And it's only since one century that we know that gravitation is due to defomation of space-time...
So, from the mere observation of what is the case, we have come to an explanation.
Could it be the case that the same occurs with kamma?