The Universe is all of space and time (spacetime) and its contents which includes planets, moons, minor planets, stars, galaxies, the contents of intergalactic space and all matter and energy. The size of the entire Universe is still unknown.
The Universe can be defined as everything that exists, everything that has existed, and everything that will exist.
According to our current understanding, the Universe consists of spacetime, forms of energy (including electromagnetic radiation and matter), and the physical laws that relate them.
The Universe encompasses all of life, all of history, and some philosophers and scientists suggest that it even encompasses ideas such as mathematics and logic.
In a nutshell
The most popular theory of our universe’s origin centers on a cosmic cataclysm unmatched in all of history – the big bang. This theory was born of the observation that other galaxies are moving away from our own at great speed, in all directions, as if they had all been propelled by an ancient explosive force.
The big bang
Before the big bang, scientists believe the entire vastness of the observable universe, including all of its matter and radiation, was compressed into a hot, dense mass just a few millimeters across. This nearly incomprehensible state is theorized to have existed for just a fraction of the first second of time.
Big bang proponents suggest that some 10 billion to 20 billion years ago, a massive blast allowed all the universe’s known matter and energy – even space and time themselves – to spring from some ancient and unknown type of energy.
The theory maintains that, in the instant – a trillion-trillionth of a second – after the big bang, the universe expanded with incomprehensible speed from its pebble-size origin to astronomical scope. Expansion has apparently continued, but much more slowly, over the ensuing billions of years.
Scientists can’t be sure exactly how the universe evolved after the big bang. Many believe that as time passed and matter cooled, more diverse kinds of atoms began to form, and they eventually condensed into the stars and galaxies of our present universe.
Later in the Universe