All existing matter, space & other phenomena constitute the Universe. In other words, everything that exists anywhere is included in the universe.
The Solar System includes dozens of moons and countless pieces of rocky & icy debris along with planets. The Sun is one of the hundreds of billions of stars that form our galaxy, the Milky Way. There are several hundreds of billions of galaxies in the Universe, and scientists see them in every direction of the sky.
How was Universe created?
Big Bang Theory
The big bang theory proposes that the universe was once extremely compact, dense, and hot. Some original event, a cosmic explosion called the big bang, occurred about 13.7 billion (1.37 x 1010) years ago, and the universe has since been expanding and cooling.
According to the big bang theory, the universe expanded rapidly in its first microseconds. A single force existed at the beginning of the universe, and as the universe expanded and cooled, this force separated into those we know today: gravity, electromagnetism, the strong nuclear force, and the weak nuclear force.
Intelligent Design Theory
Some scientists say that the Big Bang theory cannot explain everything. The Universe is too perfect to have come into existence by chance. So, they propose that there has to be an Infinite Intelligence who has designed the Universe. God has created this Universe. This theory is known as Intelligent Design Theory.
Several scientists, including the Georges Lemaitre in Belgium, Willem de Sitter in Holland, and Alexander Friedmann in Russia, gave different theories, and the universes described by the different solutions varied. De Sitter’s model had no matter in it. This model is actually not a bad approximation since the average density of the universe is extremely low. Lemaitre’s universe expanded from a “primeval atom.” Friedmann’s universe also expanded from a very dense clump of matter, but did not involve the cosmological constant. These models explained how the universe behaved shortly after its creation, but there was still no satisfactory explanation for the beginning of the universe.
Formation of Solar System
Many scientists favor the planetesimal theory for how the Earth and other planets formed out of this solar nebula. This theory helps explain why the inner planets became rocky while the outer planets, except for Pluto, are made up mostly of gases. The theory also explains why all of the planets orbit the Sun in the same plane.
Within the planetesimal Earth, heavier matter sank to the center and lighter matter rose toward the surface. Most scientists believe that Earth was never truly molten and that this transfer of matter took place in the solid state. Much of the matter that went toward the center contained radioactive material, an important source of Earth’s internal heat. As heavier material moved inward, lighter material moved outward, the planet became layered, and the layers of the core and mantle were formed.
Our solar system began forming about 4.567 billion (4.567 x 109) years ago, when a cloud of gas and dust between the stars in our Milky Way Galaxy began contracting. The gravitational energy caused by this contraction heated the solar nebula. As the cloud became smaller, it began to spin faster, much as a spinning skater will spin faster by pulling in his or her arms. This spin kept the nebula from forming a sphere; instead, it settled into a disk of gas and dust.
In this disk, small regions of gas and dust began to draw closer and stick together. The objects that resulted, which were usually less than 500 km across, are the planetesimals. Eventually, some planetesimals stuck together and grew to form the planets. Today, there are thousands of Asteroids, innumerable Comets, hundreds of moons and about 12 Planets in the Solar System.
Formation of Earth
During Earth’s early formation, some 4.567 billion years ago, it formed and was enlarged by collision with many smaller bodies, including asteroids, meteorites, and comets that crashed into the early planet at high velocity. When a large asteroid comes to a halt in collision with Earth, the asteroid’s energy is released as heat and is stored within the planet. One such giant impact occurred early in Earth’s history, when a Mars-sized object struck Earth and tore off a piece that is now the Moon.