A black hole is a place in space where gravity pulls so much that even light can not get out. The gravity is so strong because matter has been squeezed into a tiny space. This can happen when a star is dying. Stars with masses 20 times the mass of our sun could form a black hole.
Two points are very important here.
1) Black holes have a huge mass and huge gravity.
2) Stars with masses of about 20 times mass of sun forms black holes
During the Big Bang, the entire mass of universe came together.
Can you guess how much it is?
It is billions and billions and billions more than the mass of the biggest star.
What would happen when such a huge mass concentrates?
1) Mass would shoot up.
2) Gravity will shoot up too.
What happens when gravity increases?
Well, everything has an equal and opposite reaction. So, if gravity pulls, there should be another force, which equally pushes particles away.
The push(due to gravity) and pull ( reaction force oposite to gravi) force stability follows a particular order.
For small stars, when they die, the push becomes greater than gravitational pull. So, instead of becoming black holes, they burst into supernovas.
For larger stars, upto 20 times our sun, the gravity is more effective, and exceeds the push.
So, they become black holes.
For much larger masses, like that of entire universe, it again reverses, the push is much larger than pull of gravity.
This intense push will cause it to burst, and thus big bang formed.
Now, I told earlier that action of gravity and reaction is equal, according to newtons law. But, how is it possible for push and pull forces to be Greater than the other?
For this, you will need to understand Einstein's law , where E=mc^2.
This means, mass can form energy, and energy can form mass.
In our universe, a small amount of this interconversion occurs. Whichever is relevant dominates. Sometimes, the gravity pull (energy) may form mass , then gravity becomes less than reaction force, and star bursts.
If mass of stars become gravitational energy, then the gravity will exceed, forming black hole.