Big Bang Theory was originated in 1931 by Georges Lemaître. Further improvements in this theory were made in 1940’s by George Gamow with his students Ralph Alpher and Robert Herman. Other theories were also derived, in order to explain the origin of the universe, one of them was Fred Hoyle's "steady-state" theory, but only the big bang theory came close to the appropriate number of important explanations about the origin of the universe. In a survey which was conducted in 1959 on scientists across the whole America and they were asked only one specific question “What do you think about the age of the Universe?” Among all the scientists in America more than two said that there was no start or origin of the Universe. Those two third scientists of America believed that the Universe was eternal. Now most scientists think that the universe came into being from singularity.
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Edwin Hubble in 1929 discovered that the Universe is expanding at high speed and this discovery was revolutionary. Edwin Hubble noted that distances between the galaxies outside the Milky Way is increasing and galaxies are moving away from us. Edwin Hubble rapidly understood that it means there must have been an instant in time and this is now acknowledged to be about 14 billion years ago when the whole universe was at a single point known as the singularity. Astronomers combine mathematical models with observations in order to develop effective theories of how the Universe started expanding. Albert Einstein's general theory of relativity alongside with standard theories of fundamental particles helped a lot to explain the Big Bang Theory.
The laws of physics were not obeyed by the early Universe as we know them today and we cannot guess with great accurateness what the Universe looked like during the first minutes of creation but the scientists have been able to explain how the Universe evolved. According to Big Bang Theory if we were able to look at the Universe, just one second after the Big Bang happened, what we can see is a 10-billion degree ocean of, protons, neutrons, photons, neutrinos, electrons and positrons (anti-electron).
After that we would see the Universe cooling down and neutrons either decaying into combining with protons to make deuterium or protons and electrons. As it continued to cool, we would see that it would finally reach the temperature where the formation of neutral atoms occurred by electrons combined with nuclei. In 1996, observations of very far supernovae changed the picture of Big Bang Theory in our minds. It had constantly been supposed that the matter in the universe would probably slow its rate of expansion. Mass creates gravity and the property of gravity is; it pulls everything towards itself and thus pulling may cause slow of the rate of expansion of the universe, but recent observations showed that rather than slowing down the rate of expansion is increasing.
So far we do not know what actually is causing this so in order to explain this, we introduced another term of “Dark Matter and Dark Energy”. We simply don't know what Dark Matter and Dark Energy actually is but we can observe its effect in the universe mostly in the form of increasing the rate of expansion of the universe. Dark energy is a type of dynamical fluid, which is unknown to physics, or maybe it is a property of the empty space. Dark matter and Dark energy are now the most interesting fields of research because we know nothing about it.
In this article, I have made some attempts to explain the origin of the universe. It is true that we will never actually know how it initiated with 100% accuracy. We can only wonder and give our best guesses about the origin of the universe.