Constancy of light speed may not be true

Every mass defect creates a super thin form of matter, and the entire universe is filled with it. As a form of matter, it will be attracted by gravity. So every star and planet maintains a dense region of it. The density will be more near a body and decreases with distance. In fact, massive bodies in the universe have a density gradient of this matter around them. So when light travels from one point in the universe to another, light speed will change multiple times.