Quote from kut2k2:
Don't say "we don't understand gravity" when what you really mean is you don't understand gravity. You didn't know what the word "gravity" meant until somebody explained it to you.
And the reason we say gravity exists is because it does exist: the evidence for gravity is overwhelming. As I posted elsewhere, even infants are aware of what we call gravity, whether they know it by that name or not.
Isaac Newton defined gravity quite nicely, and he measured it as well. It turns out his definition didn't explain all later observable astronomical phenomena, and Einstein came along with an improved theory about gravity called general relativity that covered the data that Newtonian gravity couldn't account for. But Newtonian gravity is still perfectly adequate for explaining the vast majority of gravitational events encountered by humans.
Sometimes a scientific theory is not so much replaced as it is subsumed by a better theory. If the earlier theory had no validity at all, it would never have gained much traction in the first place.
For non-relativistic events, Newton's theory of gravity works just as well as Einstein's, and has the advantage of being much simpler. Newtonian physics got us to the moon and back.
Thats sad that you think so shallow.