About 13.8 billion years ago, all the matter and energy in the universe was packed into an unbelievably small, hot, dense state. Then - for reasons physics is still working out - it started expanding. Rapidly. The Big Bang isn’t the story of an explosion in space; it’s the story of space itself starting to grow.
In the first tiny fractions of a second, the universe inflated to enormous size. Within a few minutes, it had cooled enough for the first atomic nuclei (hydrogen and helium) to form. After about 400,000 years, it cooled enough for atoms to hold together, and light could finally travel through space without bouncing off charged particles. Stars and galaxies started forming a few hundred million years later.
We have direct evidence of this. The Big Bang’s leftover heat is still around - a faint, uniform microwave glow filling all of space, called the cosmic microwave background. It was first detected by accident in 1964. Astronomers can map it in detail with satellites; the small variations in its temperature show the seeds from which all the structures of today’s universe grew.
