First, the difference, when talking about space, between "infinite space" and "space without limits". In my mind and I do believe in common parlance they are the same concept. I personally cannot conceive a "limit" to the universe so made that there is nothing "after" or "beyond" that limit. Not even, that is, the concept of "being empty". My mind - and I guess yours, really - cannot conceive such a thing. A sphere to me can always be inscribed in a cube. You think "a sphere" because you imagine its dimensional limits and somewhere something in which those dimensional limits exist.
Regarding gravity, of course it was observed by man since he was a monkey, with or without a Leica. Everybody observe that they fall. "Gravity" litterally means "being heavy". Weight is something that men, and animals, experience and know very well without need to go to University, or Elementary school for that matter.
Galileo gave us the law of gravity stating "weight" being the product of mass x speed of "fall", and showed us that "speed of fall" is independent from mass, and has uniform acceleration of 9.8 m/s*s. This equation is the base of the science known as ballistic and in general his equation is somehow everywhere in our scientific world. He naturally gave many other important contributions to mankind, stating various laws about motion, inertia, motion quantity and other concept which I forgot since long but that are at the base of our technology.
I would certainly not debate about the relative merits of Galilei and Newton. Newton was born just after Galilei died. Newton built upon what Galilei built. They are a giant on the shoulders of another giant. Newton gave us the laws of attraction of masses in the general case. Galilei gave us the law of attraction of masses when one mass (the Earth) is so hugely superior to the other mass (the "grave", and object with a weight) the the mass of the latter is negligible. Newton gave a generalization of Galilei laws, the law of reciprocal attraction of two masses (planets, stars). They are both giants and I don't see how can one think about ranking them. Newton, I am sure, would have never thought of himself as "better" than Galilei, and Einstein (or whoever) would never think of himself as "better" than Newton.
As Leonardo da Vinci said, "a poor disciple is he who doesn't overcome his master". The disciple "starts" by having all the knowledge of his master. It is his duty to add to that.
Both Newton and Galilei gave convincing repeatable demonstrations of their equations. Their scientific discoveries form the base of all our technological progress of the last centuries.
Most of those last century's hypothesis about the universe are, on the other hand, as far as I know, resting entirely on mathematics. We have plenty of astronomical theories that do not have, possibly do not seek by evident impossibility to seek, an empirical test. They can be legitimately called "theories" or "hypothesis". Possibly brilliant and genial ones. But being confined in the realm of the hypothesis, I would not use the indicative mode of any language to express their thesis. Indicative is the mode of certitude.