What is the theory of the beginning of the Universe?

The best-supported theory of our universe's origin centers on an event known as 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  was the moment 13.8 billion years ago when the universe began as a tiny, dense, fireball that exploded. Most astronomers use the Big Bang theory to explain how the universe began. But what caused this explosion in the first place is still a mystery.

Since the Big Bang, the universe has been expanding. In the early years, everything was made of gas. This gas, mostly hydrogen and helium, expanded and cooled. Over billions of years, gravity caused gas and dust to form galaxies, stars , planets, and more.

The matter that spread out from the Big Bang developed into everything in the universe, including you. You are made of star stuff!

How and when did the universe begin? No other scientific question is more fundamental or provokes such spirited debate among researchers. After all, no one was around when the universe began, so who can say what really happened? The best that scientists can do is work out the most foolproof theory, backed up by observations of the universe. The trouble is, so far, no one has come up with an absolutely indisputable explanation of how the cosmos came to be.

Since the early part of the 1900s, one explanation of the origin and fate of the universe, the Big Bang theory, has dominated the discussion. Proponents of the Big Bang maintain that, between 13 billion and 15 billion years ago, all the matter and energy in the known cosmos was crammed into a tiny, compact point. In fact, according to this theory, matter and energy back then were the same thing, and it was impossible to distinguish one from the other.

So if all the galaxies are moving away from Earth, does that mean Earth is at the center of the universe? The very vortex of the Big Bang? At first glance, it would seem so. But astrophysicists use a clever analogy to explain why it isn't. Imagine the universe as a cake full of raisins sitting in an oven. As the cake is baked and rises, it expands. The raisins inside begin to spread apart from each other. If you could select one raisin from which to look at the others, you'd notice that they were all moving away from your special raisin. It wouldn't matter which raisin you picked, because all the raisins are getting farther apart from each other as the cake expands. What's more, the raisins farthest away would be moving away the fastest, because there'd be more cake to expand between your raisin and these distant ones.

That's how it is with the universe, say Big Bang theorists. Since the Big Bang explosion, they reason, the universe has been expanding. Space itself is expanding, just as the cake expanded between the raisins in their analogy. No matter whether you're looking from Earth or from an alien planet billions of miles away, all other galaxies are moving away from you as space expands. Galaxies farther from you move faster away from you, because there's more space expanding between you and those galaxies. That's how Big Bang theorists explain why light from the more distant galaxies is shifted farther to the red end of the spectrum. In fact, most astronomers now use this rule, known as Hubble's law, to measure the distance of an object from Earth—the bigger the redshift, the more distant the object.

In 1965 two scientists made a blockbuster discovery that solidified the Big Bang theory. Arno Penzias and Robert Wilson of Bell Telephone Laboratories detected faint microwave radiation that came from all points of the sky. They and other physicists theorized that they were seeing the afterglow from the Big Bang's explosion. Since the Big Bang affected the entire universe at the same moment in time, the afterglow should permeate the entire universe and could be detected no matter what direction you looked. This afterglow is called the cosmic background radiation. Its wavelength and uniformity fit nicely with other astronomers' mathematical calculations about the Big Bang.

Explanation:

I hope it will help you