Because the Standard model is undergoing major repair right now. Yes, gravitation is a THEORY insofar as Einstein’s theory of general relativity goes, but it seems to hold true over the entirety of the observable universe and brother, let me tell you, a Hubble volume is fucking BIG. The search for a satisfactory theory of gravity that is compatible with our current understanding of quantum particle physics and the metrics in general relativity is THE ongoing quest in physics right now, the unified field theory, the golden apple. The problem is that most of the theories we can come up with are extremely hard to test. Take string theory, which needs a particle collider that would make the LHC look like a popgun to be proven across all energy scales. We know that is likely incorrect, as we can now use LHC results to set boundaries on dimensionalities predicted by string theory, and the eleven dimension hypothesis so touted by string theorists has been reduced to a heap of meaningless calculations in light of the new results. That being said, great strides are also being taken in this area. I read today an article that mentioned a mass boundary being set on dark matter particles, which is a huge leap forward as dark matter may well prove integral to our understanding of how gravity works on a quantum scale. That being said, physics and math are HARD subjects. And like every hard subject, you need time before the truth comes exploding out.
