yes, it's time. not mass. but there is some relationship between time and mass. i mean, refraction works on this same principle too, newton's prism rainbow happens because different frequencies of light slow down going through the material. so there is also that at play as well, just to make it even more insanely incomprehensible. probably basically the light must be stopping to wait for atoms to get out of the way, due to the harmonics between the spins of the matter particles and the light. so i'm not sure if that is exactly the same thing, i can't tell, is it a scale thing? light travels slower through dense objects as well, so could it be that the light is experiencing time distortion?