After watching Lawrence Krauss' lecture 'life, universe, and nothing', an interesting idea lighted up in my mind. Is it possible that dark matter is just spread out matter waves and dark energy extremely long wavelength light?
matter -> expansion -> dark matter (delocalized matter waves/huge atoms) visible and xray radiation -> expansion -> invisible light with wavelength greater than the diameter of universe (perhaps the source of energy of the empty space?!!!!)
so dark matter today may be what we will become in the future. and we are what dark matter was in the past. eternal expansion just spreads out everything but new stuff is created all the time. anyone living in such a universe at any point of time will see the universe almost as we see it today, expanding with a beginning.