I think it's because the expansion of the universe is happening faster than the speed of light. So the light from the far regions can never reach us. The unive

I think it's because the expansion of the universe is happening faster than the speed of light. So the light from the far regions can never reach us. The universe could well be infinite, but the Hubble volume is pretty much finite... If it was only dust, it alone would not explain the darkness due to re-emission of light (as other commenters pointed out).