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The answer depends on if you're looking for a classical answer or a quantum answer. As far as a quantum answer goes, I haven't taken quantum optics yet, so I can't really give you a good answer. I can make up some BS about atoms absorbing a photon and reemiting the photon, thus causing it to take more time to travel the total distance, but I don't think this answer is right.
Classically, think about the atom as a positively charged ion attached to a negatively charged valence electron(s) with a spring. Light is an oscillating electric field. The electric field pulls on the electron and pushes on the positive core. (Or the other way around). Because the light-electric-field oscillates, this causes the electron-core system to also oscillate. Of couse, the oscillation of the electron-core system causes another oscillating electric field. This field interacts with the original light-field. The two effect each other to such a degree that they are pretty much indistinguishable from each other.
Looking at this problem mathematically, the presence of a material (the dipole created in the material) changes several constants, namely the permeativity and permeability, in the wave equation which describes how light travels. These constants are used to derive the "speed-of-light" and since they have changed from their freespace values, the speed of light in a material is different from its free-space value.
So long story short, the speed of light is usually slower going through a material than not. The speed is only incidentally related to the density of the material.
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Then what's to stop us, pretty baby. But What Is And What Should Never Be.
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