The two main theories of how the universe came to be are the Steady State Theory and the Big Bang Theory.
The Steady State Theory:
The Steady State theory was proposed in the late 1940’s by Hermann Bondi, Thomas Gold and Sir Fred Hoyle. Their theory stated that the universe had no beginning and no end and that it looked the same from any point or any time, any place in the universe. The advantage of the Steady State theory over other theories was its simple explanations of certain topics. For example, to account for the decrease in density that would result from expansion, the Steady State theory claimed new matter must be constantly created in order to maintain a constant density. Furthermore, since the universe is unchanging over time, the universe needs no explanation of its beginning.
Belief in the Steady State Theory began to disappear in the 1960s after the discovery of quasars. A quasar is a luminous galactic core, powered by a massive black hole. Quasars are so very far away from Earth that their light takes several billion years to reach the planet. Quasars are objects from the past which indicate that a few billion years ago the structure of the universe was very different from how it is today. They have very large redshift (“redshift” occurs when stars are moving rapidly away from Earth causing the light from the object to be viewed red) which also shows that the universe is expanding. Astronomers also found that the universe actually evolves over time. The final 'blow' to the Steady State Theory was found by radio astronomers Arno Penzias and Robert Wilson. The scientists discovered Cosmic Microwave Background (CMB) which is thought to be leftover radiation from the Big Bang. The Steady State Theory could not give a convincing explanation for CMB and so many people stopped supporting this theory.
_ The Big Bang Theory:
The Big Bang Theory was first proposed by Georges Lemaître in 1927. Our galaxy, the Milky Way, is one of about thirty neighboring galaxies in a mass called the Local Group. Astronomers found that these other galaxies were moving away from us along with others ones found deeper in space. They then propose that all matter in the universe must have come from one source which, at the beginning of time, exploded sending this matter out in all directions. This theory was called the Big Bang Theory. The model suggests that about 10 billion years ago, that all matter was found inside one huge, hot, dense ball of radiation and subatomic particles. An explosion then took place and the matter expanded. As it expanded and cooled, protons, electrons and neutrons were formed. After further expansion, gaseous atoms like hydrogen and helium were formed. The matter that had been thrown out by the Big Bang was denser in some areas creating, in these parts, gravitational forces which squeezed particles of matter together forming galaxies containing stars. Astronomers predicted that small amounts of matter and radiation would have been trapped in the spaces between galaxies as a result of the explosion. Cosmic Background Explorer satellite (COBE) and recently the Hubble Space Telescope have detected these small amounts of radiation and matter in intergalactic space, where before thought to have contained nothing.