I just learned from an Astronomy course that when the sub atomic particles were not bound together, they were moving around freely, but after the universe expanded and cooled, they began to combine to form Hydrogen atoms. Now, before this combination period, the photons couldn't pass through those particles, but after they did, they could. What I don't understand is why? I've read the introductory part here, https://en.wikipedia.org/wiki/Recombination_(cosmology), but I couldn't understand much. Any help?