Not so. You can very easily prove that gravity exists. You can observe it, without fail, with any number of different test. You can predict exactly where it will be found, and exactly how much of it will be found there. And your predictions will be found to be accurate each and every time. And all of this with great precison.
Not that anyone needs an example, but I learned something intersting from a co-worker whose daughter works at JPL on some of the missions to Mars. One of her functions is to calculate the gravitational effects of the rotation of the Earth's liquid core at the time of launch, and how that is going to change the trajectory of the mission package.