The Big Bang, as many cosmologists like to point out, was not very banging. Nothing exploded. But that doesn’t mean it wasn’t a busy and exciting event. The rush of inflation was a powerful outpouring of energy, which certainly included light. And yet, the energy contained in that early universe was such that light couldn’t even escape. For light to be seen, by telescopes or eyes of any species, it needs to be able to travel from its source to the observer. But the universe when it first existed was too clogged by excited particles for light to get through. Imagine the infant universe as the first pinball wizard, batting photons back and forth and around, never letting them simply fall to stream freely away into space. The paddles and levers in this case were dense seas of electrons, clogging up the cosmos by constantly absorbing and re-emitting light in random directions, keeping the cosmos in the dark.
The universe in its infancy wasn’t the well-ordered system it is today. These days, matter clumps. Yes, into stars and asteroids, but also into much smaller units like molecules and even neutral atoms; the familiar model of a positive nucleus surrounded by orbiting electrons.
When the universe first formed however, this hierarchy didn’t yet exist. Electrons and protons flowed freely in a particle soup jostling with spare energy. It took the cosmos something like a quarter to a half million years to calm down enough for protons and electrons to pair up. Cosmologists call this era
recombination, but there’s nothing “re-” about it. It was the first time ever that elementary particles joined up to form atoms.
Once they did, the pinball game stopped, and light was allowed to stream freely through the universe, which finally consisted of the empty space so prevalent today. But there was only one light present in those days: the crackle of energy from the Big Bang, already ancient but only then allowed to travel for the first time. The universe had just managed to stick together enough to form atoms, and shining stars were still eons away.
And so, after the Big Bang’s brilliance faded the universe went dark again, for something like 400 million years.