The cosmic microwave background, or CMB, is the faint afterglow of the Big Bang, released about 380,000 years after the universe began when it cooled enough for atoms to form and light to travel freely. Now stretched into microwaves by cosmic expansion, it bathes the whole sky at a temperature just above absolute zero. Tiny fluctuations in it map the seeds of all later structure and anchor our measurements of the universe's age, contents, and geometry.