I've always thought that Einstein's theory of general relativity was quite beautiful, it explained gravity as merely an intertial tendency of objects to move in straight lines (albeit different kinds of straight lines defined to be paths of greatest proper time) in a curved space-time. This explains why it's so distant from other forces in the standard model, because it's really a result of geometry. In what way do we still not understand the mechanism of gravity? It would seem that this is quite explanatory. I do not doubt the claims that we don't quite understand it, as our knowledge of something is never entirely certain, but objects travel in straight lines seems to be a good enough reasons for the effects of the so called "gravity". Is it just a stagnant philosophical struggle that we do not quite understand gravity, or is there more to this claim?