i read an article from "Astronomy Collectors Edition" book on the cosmos. It mentions in its article on inflation that about 380000 years after the big bang, radation and matter decouppled or "unmixed". what does it mean by this? before this "unmixing" , was this the dark period of the universe during which there was no light?
Not dark no, just foggy. Before matter managed to properly clump together in atoms the universe was a sea of elementary particles and photons of light and the whole lot was constantly bumping into each other so that it was not possible for light to travel in a straight line and hence all you would see would be an even white haze everywhere. When the matter clumped into atom the light could move around freely and hence we see frozen in the CMBR the state of the universe when matter and light un-mixed.
  