3 minutes after the Big Bang
About 3 minutes after the Big Bang, the Universe was still at a temperature of aroung a billion degrees. By this time, however, the matter which makes up the Universe was fixed: about three quarters hydrogen and one quarter helium. We still see basically this proportion today, though it has changed slightly over time as stars have burnt hydrogen and helium and made heavier elements.
The Universe was still opaque, and the vast majority of it is still composed of photons, or particles of light, scattering around. For the next few hundred thousand years, the Universe will remain this way. Over time, the slightly denser regions of the Universe will attract more material from the less dense regions, making the Universe ever so slightly non-uniform.