Newton is considered to be legend in physics. He was an intelligent person for sure who came up with differential calculus. This is the science of examining how fast something is changing and how to use this information to calculate more interesting things. For example if something is moving, the speed is how fast it is moving. And the rate of change of the speed is the acceleration. Beautiful maths.
He gave the physicians the mathematical tools to examine the world and one day as the story says newton got an apple in his head and he said, enough, lets understand gravity. So he started throwing things and doing measurements. Out of all these measurements he saw that things fall with constant acceleration and when he put his data on graphs he saw that they all thing fall with the same acceleration, the world famous G in the f=GmM/r^2 and brand it as the newtons law of gravity.
That was very convincing at the time because everyone in the earth who repeated the experiments came with agreement with him, and that lead to the birthplace of Gravity constant and the gravity law !
Years later, the humans managed to send spaceships out of the space confident that they could calculate their trajectories using this law. In space you only have planets so to our belief our calculations should be accurate. When the Voyager space mission started going very far things didn't work out at all with the calculations they where moving with different speeds than expected.
But it wasn't only spaceships that didn't like newtons law. Astronomers started to measure the rotation speed of galaxies. Spectroscopy lets you know how much material there is on a galaxy and only with this you should be able to calculate the centripetal force that keeps things spinning on a galaxy. Galaxies where rotating faster.
With no explanation to these, and because we couldn't doubt newtons law scientist had to come with some explanation. So they said ... in order for the equations to work right there must be more mass that we can't see and called that "Dark matter". People love magic and they soon discovered that since there is dark matter there must be "dark energy" as well.
Hot covers on magazines and crazy funding for mad ideas. To cut the story short. Newtons law is totally wrong because we have no idea yet about what is the nature of gravity or mass. But, we would never doubt Newtons law.