The Universe is everything that exists—space and all the stars, planets, and other matter it contains. Astronomers believe that a huge explosion called the BIG BANG created the Universe.
Between groups of galaxies, most of the Universe seems empty, but it is full of a mysterious dark energy and radiation such as light and radio waves. The stars, nebulas, and planets in galaxies are made of ordinary matter, but galaxies are also surrounded by vast amounts of invisible DARK MATTER. Four basic forces control the Universe—electromagnetism, the weak force, the strong force, and gravity.
The Universe is bigger than we can possibly ever see or imagine. Astronomers can now spot objects more than 12 billion light-years away from Earth, or an incredible 70 billion trillion miles (115 billion trillion km) from our planet.
Astronomers believe that a giant explosion, the Big Bang, created the Universe about 14 billion years ago. Before that, there was nothing—no matter, no space, and no time. The Universe began expanding at the Big Bang, and it is still expanding.
In a fraction of a second, the newborn Universe grew from the size of an atom to a searingly hot fireball bigger than a galaxy. As it spread out and cooled, it formed a thick soup of tiny particles of matter. It took another 300,000 years for the first atoms to appear.
We know dark matter exists only because its gravity pulls on stars and galaxies and bends light rays. No one has discovered what it is made of. There is ten times more dark matter than ordinary matter.