What were the universe's dark ages? 

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Soon after the universe was born, the universe was plunged into the dark ages. That's because no sources of light remained to illuminate the expanding universe. 

But slowly, atoms accumulated into larger structures, molecules, then clouds of gas, then stars, and eventually, galaxies. Light from these new stars and galaxies burst forth into their surroundings, and these photons began stripping electrons off atoms, reionizing them. These freely moving photons brought an end to the dark ages, lighting up the universe, and heralding in the age of reionization. Yet, it's still a mystery how long the dark ages lasted, and exactly when reionization began. 

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