I'm trying to understand on how people came up with gravitational law i.e. observations, experiments, theories etc. I have read a bit but there are some blanks that I cannot fill. I need some help regarding that. It seems that people like Hooke, Newton already knew that there exists gravitational force between every two objects. And that it is proportional to their masses and inversely proportional to square of distance between them. This was confirmed by Newton based on Kepler's laws. But I want to know how did they know this before Kepler's laws? What experiments did they do to come up with this? How did they come up with the concept of mass in the first place?