Antimatter!

Sounds like something that would only come from a strange Sci-Fi movie right? Well you're wrong! Antimatter was created when matter was created at the Big Bang. Antimatter has been defined as the mirror image of matter.So when matter was created, antimatter was created at the exact same time. Imagine a hot metal sheet in a coin factory ('energy'). When you stamp out a coin from a metal sheet, you are left with a coin and a hole in the sheet.You could call this hole an "anticoin". But, you cannot bring matter and antimatter together because when you do, burst back into energy, like putting the coin back into the hole in the sheet. So at the beginning of the Universe, these particles had an all out war against each other and the matter and antimatter annihilating each other.
Why is the Universe not just a burst of energy then you ask? Well for reasons still unknown to scientists, at the beginning, there was a tad bit more matter than antimatter which allowed the matter to overcome the antimatter and when the Universe cooled down enough so that no more matter or antimatter could be created, matter was all that was left and continued to expand and create the universe you see today. The result of the battle can be seen still today because the leftover radiation can be seen all throughout the Universe. This radiation is called Cosmic Background Radiation.

And It All Started With A Big Bang!

So as the insanely catchy theme song for the sitcom "Big Bang Theory" states, the whole universe started in a hot dense state. Then, everything had changed. Because it is impossible to try and recreate the big bang in a lab, scientists can only theorize about what had happened. This theory came to be because of the Hubble telescope. Through this telescope, scientists were able to see that entire galaxies were moving away from us at a very rapid speed. The explosion from the big bang could have only been the only thing that would have caused that. The most amazing thing is that the explosion was only there for a very short time. In fact it only took about one second for the building blocks of the universe to form.

This picture calls that the Big Freeze Out. Then, 12-15 billion years later, our Universe as we see it today is in existence.