Newton was against his own theory of gravity. He couldn’t accept an invisible force acting on everything. However, his math kept coming back to a force being there. He could explain how this force worked. He just couldn’t explain what it was, and to this day we still can’t fully.
The final test of his theory was explaining the tide. He did so by coming up with a simplified model of the earth, involving only the moon, no sun, and two islands: one island at the top and one directly facing the moon.
As the moon applies it’s gravitational force to the ocean closest to it, a centrifugal force, due to rotation, is pulling the ocean on the opposite side away. This gives the ocean an elliptical shape. With the moon position closest to the island on the right, it exposes more of the island at the top and less of the island facing the moon. Thus showing how the moon causes a high tide at one location and a low tide at another.
This basic model grows more complex with the tilt of the earth, the motion of the moon itself, and the addition of the sun. But a good starting point is the simple model. This ability to break down a problem to the important pieces that explain the essence without unnecessary complexity, made Newton’s theories stick.