The basic and most widely accepted theory of the origin of the universe is the Big Bang theory. ¹⁾

The big bang theory assumes that the universe began at a single point the size of a pinhead, which as a result of a huge explosion began to expand and is still expanding today. One piece of evidence that supports the big bang theory and the expansion of the universe is the fact that the farther away from earth the observed galaxy is, the faster it is moving away from earth. ²⁾

After an initial period of rapid expansion, the universe began to cool. ³⁾

As the temperature dropped, subatomic particles and then atoms separated. The universe was filled with primordial matter - gaseous clouds consisting mainly of hydrogen and a small amount of helium and lithium. Under the influence of gravity, the first atoms began to move closer together and form denser clusters, giving rise to the first stars and galaxies. ⁴⁾

The observable universe is estimated to be 13.8 billion years old. ⁵⁾

The development of the universe can be divided into five stages. ⁶⁾

The early universe lasted from 1 picosecond for 370,000 years. ⁷⁾

The next stage was the Dark Ages and the formation of large cosmic structures. This period lasted from 370,000 to 1 billion years after the big bang. During this time, matter in the universe was grouping together under the influence of gravity. New stars were formed and merged into larger structures. ⁸⁾

We are currently living in the fourth stage of the universe's development. It resembles the one 12.8 billion years ago and will continue to look that way for the next billion years. ⁹⁾

There will eventually come a point in the history of the universe when new stars will stop forming. It will have expanded to such an extent that it will become impossible to concentrate enough hydrogen clouds to form a star. The extended universe will be so vast that the only observable part of it will be the home galaxy and its immediate surroundings, beyond which the observer will be surrounded only by a perfectly black cosmic void. ¹⁰⁾

There are about 200 trillion galaxies in the universe. Each has a different composition and size. The largest galaxies can span up to 2 million light-years. ¹¹⁾

The largest galaxy that has been spotted so far is IC 1101. ¹²⁾

The smallest galaxy discovered so far is Segue 2, located adjacent to the Milky Way. ¹³⁾

The exact size of the universe cannot be estimated. ¹⁴⁾

The Sun has a diameter of 865,374 mi. ¹⁵⁾