Re:More info here (Score 1)

Dark energy (apparently.) Which is the stuff that's accelerating the expansion of the universe. There is less of it in regions where there's no matter (its a kinda anti-gravity thing) so the photons travelling through the void do not gain as much energy as the photons that travelled through matter infested space. That's how my head got round it, anyway. Although the site mentioned photons loosing rather than failing to gain energy.