StartsWithABang writes: Just a second after the Big Bang, the Universe was a hot bath of radiation, with a small fraction of protons and neutrons in about equal numbers left over. By time it was four minutes old, it was 92% hydrogen (by number of atoms) and 8% helium. Yet the Universe has aged nearly 14 billion years since then, and have formed many generations of stars, all of which burn hydrogen into heavier elements. So how much hydrogen is left, and how much will be left far into the future? A lot more than you might think.