The discovery is especially exciting for scientists because the explosion occurred during the so-called "cosmic dark ages", which started a mere 400,000 years after the Big Bang set the Universe in motion some 13.7 billion years ago. During this period, free electrons and protons combined to form neutral atoms with the same number of positive and negative charges, resulting in an opaque -- or "dark" -- universe. Not until 800 to 900 million years after the Big Bang were atoms and molecules "re-ionised", or electrically charged, resulting in the relatively transluscent inter-galactic medium we see today. Thus evening came, and morning followed, the first day.