Early Stages of the Universe 

I guess you all must have heard this or read somewhere that in the very beginning, the universe was tightly packed into a point of infinite density called 'singularity' which then exploded into the universe. This idea is given by The Big Bang Theory. 
But the astrophysicists say that this model of the universe's creation is wrong. There was no such explosion as in a volcano where all the rocks go flying away. There were no stars or planets or even atoms that were ejected instead the entire universe expanded itself & became larger. Atoms only coalesced million years later as the temperature cooled & bigger structures were formed even later. 
The Big Bang didn't happen when the universe was a singularity but when it was very hot & dense & it didn't occur at a definite point in space but everywhere in the universe simultaneously & contradicting the belief, The Big Bang isn't a theory of the creation of the universe. Currently, there's not ample research that can tell us how or why was the universe formed or what caused the inflation.
The Big Bang model gives an account of the events that happened after the inflation which led the universe to look like it's today. It has been verified with proof that can be seen even after 13.8 billion years. But, before the 1980s this model failed to explain some of the aspects of the universe like its homogeneity, flat geometry & absence of magnetic monopoles. 
To complete the inadequacies of The Big Bang Theory, The Cosmic Inflation Theory was introduced by Alan Guth & others in 1981 & it is now considered a definitive part of early Big Bang cosmology. In simple terms, the theory states that the universe expanded exponentially fast, even faster than light from 10^-36 sec after the beginning to 10^-32 sec. During this period it expanded to a factor of 10^78 m^3 from a tiny point. 
According to Einstein's Theory of Special Relativity, no object can move faster than the speed of light in space except the space itself. Some physicists believe that The Big Bang occurred about 10^-12 sec after the inflation & it is not the point of the very beginning when the time first started i.e. t=0. 

The Cosmic Microwave Background 

Let's now see how The Inflation Theory solves the three issues with The Big Bang Theory. 

1. The universe is very homogeneous & isotropic. On a large scale, it appears pretty much uniform. Let's consider that before inflation, the universe was completely random like the surface of a deflated balloon with uneven density. Different thicknesses & wrinkles were randomly distributed. When the balloon is inflated all the wrinkles get smoothed out & all the density differences get diluted. This is similar to what happened during inflation when a tiny volume got expanded by a factor of 10^78. It stretched out the fabric of space-time eliminating all the irregularities in it.
2. Inflation stretches any initial curvature of the 3D universe to near flatness. We don't know exactly if the universe is completely flat but if it is curved then it is negligible to measure due to its exponentially large size of about 93 billion light-years. Therefore it appears flat to us. 
3. Monopoles can be formed theoretically only at very high temperatures similar to that present at the time of The Big Bang but they should be stable enough to exist. The hypothesis is that inflation quickly cooled the universe due to rapid expansion so the only monopoles that got created before the inflation would continue to exist but their density would be exponentially small as they will get scattered across a space of a factor of 10^78 m^3. This would make them next to impossible to get detected. 
Though The Inflation Theory implies that the universe is homogeneous & smooth but the Cosmic Microwave Background (CMB) shows that it is not completely. There are some anomalies, and small temperature differences called the anisotropy of CMB & this is exactly what was expected. The universe appears uniform only at a large scale due to inflation but these minute anisotropies explain the formation of gigantic structures in space. When the universe was a compact point the quantum fluctuations present in the density of matter within the point expanded to huge levels to become high-density regions which then clumped together to form stars, galaxies & superclusters. 

CONCLUSION

The Big Bang Theory is one of the greatest in science but it has some inadequacies for which The Cosmic Inflation Theory was developed to explain them. If The Inflation Theory is confirmed then one of the most fascinating things would be that our universe is just one amongst the many universes or the multiverse. Even Alan Guth himself quoted, "It's hard to build models of inflation that doesn't lead to a multiverse ". According to this theory, that stunning expansion would have created many bubbles of space-time that formed other universes having different laws of physics & our universe might be one of them. Then we'll need to search for the entire multiverse in the Cosmic Microwave Background radiation.