well this particular theory is over one hundred years old and it hasn't been disproven - don't hold your breath
loledit: and WTF no we haven't been taking his word for it. tons of people have tried to disprove him and failed.
First things first, there was a theory that the world was flat for hundreds of years, and that's a pretty substantial theory that got disproved.
And oh, my bad, he discovered why things (Moons, Earth, planets etc) stay in orbit and what causes them to orbit in the first place.
I'm not saying we've just been completely taking his word for it, it's more that he was such a brilliant mind that hardly anyone thinks there's even a need to challenge what he said. Either way like you said ''this particular theory is over one hundred years old'' and so much has changed since then, we've advanced so much, we've done so much that was thought impossible back then, and with it our theory's should change.
mostly because, y'know, nothing can go faster than light. like, at all. ever.
Yes it can, if you slow light down, which is probably cheating but still.
PS. i never said newton ''discovered'' gravity