river: No, it really isn't. Well, there are two possibilities: 1. Our understanding of gravity works pretty well, which means there's something we can't directly see at the galactic centre that has the mass of a million Suns. or 2. Our understanding of gravity is deeply flawed - so much so that we are completely mistaken about why one object orbits another object. Which do you think is more plausible? Bear in mind that we use gravitational physics successfully for all kinds of things every day.