MPEG videos of the numerical simulation of the "end of the dark ages".
God's (outside) view (26MB)
Mortal's (inside) view
(14 MB)
Explanation
Once, long-long time ago, before the first star appeared, the universe was dark and cold (quite cold, indeed, -450oF!).
Then, as first stars formed from the condensing clouds of gas, the first light in the universe was emitted (some 13 billion years ago, when the universe was only 200 million years old, a baby on a cosmic scale). However, no one would be able to see that light: the universe was opaque, as the hydrogen atoms filling the space would block any light passing through them.
But the first stars emitted not only visible light, but energetic ultra-violet light and X-rays, which are able to ionize a hydrogen atom, i.e. to strip it off its only electron, and turn the hydrogen gas into a transparent plasma. First stars, grouped in proto-galaxies, created small transparent regions around them. These regions (called scientifically HII regions; pronounced 'h-two', II is a Roman digit 2) increased in size until the neighboring regions merged together, clearing up the "fog" of neutral hydrogen, and making the universe transparent to star light, as it is now. Sir Martin Rees of Cambridge University poetically called this epoch "the end of the dark ages". Scientifically, it is called "reionization", as the hydrogen gas became ionized again (it also was ionized in the very first stages of the evolution of the universe).
The first movie above shows a simulation of mine that models cosmological reionization - the end of the dark ages. You will see the computational cube filled with brown stuff - it is neutral hydrogen gas (the color is arbitrary - I have no idea what color neutral hydrogen was 13 billion years ago). The ionized gas is glowing blue, becoming totally transparent if it is very highly ionized. Yellow dots are galaxies - they can twinkle too!
As the time goes on, you will see how first galaxies ionize and destroy neutral hydrogen around them, creating holes in the cube. Later these holes merge and the universe becomes ionized (transparent).
The second movie gives you a different prospective - in this case we sit inside a cube and see how neutral gas disappears around us - the more transparent the universe becomes, the more galaxies you can see, until the grand finale. Be patient - nothing happens for a while in the beginning, until first ionizing radiation reaches you.
Some time ago Michael Schneider, a professional journalist, wrote a very nice popular level article about the end of the dark ages and my previous work - you can access it here.