The Big Bang and the expansion of the universe

In short: According to the Big Bang theory the universe arose about 13.8 billion years ago from an extremely hot, dense state and has been expanding ever since.

In more detail: The Big Bang was not an explosion in a space but the expansion of space itself. At first there was only energy; later particles, atoms, stars and galaxies formed.

In detail

• Evidence: The cosmic microwave background (afterglow of the early universe), the abundance of light elements and the redshift of galaxies.
• Hubble: In 1929 Edwin Hubble showed that distant galaxies recede the faster, the farther away they are.
• Dark matter and dark energy: About 27 percent of the universe is dark matter, roughly 68 percent dark energy, only about 5 percent ordinary matter.
• Age and size: The observable universe has a radius of about 46 billion light-years (galaxien-und-milchstrasse).
• Open questions: What there was before the Big Bang we do not know.