When did the universe begin?

The universe began about 13.8 billion years ago (±21 million years) according to measurements of the cosmic microwave background (CMB) by the Planck satellite. The Big Bang was not an explosion in space but rather an expansion of space itself from an incredibly hot, dense state. Within the first second, fundamental forces separated and quarks formed. By 380,000 years, atoms formed and the universe became transparent — the light from this epoch is the CMB we detect today at 2.725 K. The first stars ignited about 100–200 million years after the Big Bang. Today the universe continues to expand at an accelerating rate, driven by the mysterious dark energy that makes up ~68% of total energy.