In the beginning, 10 to 20 times 10 to the ninth years ago, there was a big bang. A hundredth of a second later electrons, photons and neutrinos appeared. Three minutes later the quarks of the fundamental particles formed into light nuclei, approximately 76 percent hydrogen and 24 percent helium. 300,000 years later, and the nuclei combined with electrons to form neutral atoms, making the universe transparent to photons.
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