This law you speak of is simply your observation of it exerting an influence and determining its relationship to mass. You do not know what it is because of what it isn't... gravity is not a "thing" you can define in concrete terms the way you define any physically discernible object, such as a planet or an asteroid, or a cat...
mmmm, cats are good too. Better than humans imo... on this matter all of my colleagues are in agreement.