Why Andromeda and milky way will collide ?

In this expanding Universe why Milky way and Andromeda are likely to collide in future ?

Milky way and Andromeda
 (Image credit : NASA)

This is great question as it will decide fate of Milky way and I will try to answer it in best possible way.

When will Milky Way and Andromeda collide ?

Milky way and Andromeda  galaxy collision will decide the fate of our lives. Because when this will happen both galaxies will be destroyed.

We have nothing to worry as it will happen around 4.5 billion years later.

Why will Andromeda and Milky way will collide ?

On the smaller scales of the universe, gravity is the dominant force. We see gravity in action every day on Earth when we drop an article and the most famous example among these was the Apple which fell on Isaac Newton. These Earthly scales are actually some of the smallest scales on which gravity dominates.
On slightly larger scales, gravity is in charge of holding together the solar system, the Milky Way galaxy, and all of the nearby galaxies (known as the Local Group) including Andromeda. On scales much larger than the size of the Local Group, the gravitational attraction between two galaxy-sized objects becomes dramatically weaker (Newton’s Law of Universal Gravitation is an inverse square law), and dark energy takes over. So, what is dark energy?
The Newton's law of gravitation is a inverse square law because ;

Universal law of gravitation

Here,

•           r is distance between masses

In the late 1920s Edwin Hubble determined that the universe is expanding at an accelerating rate. This was a key discovery that even altered Albert Einstein’s view of the universe. Astronomers call the mysterious force driving the universe apart “dark energy”(NASA). Unfortunately, we don’t know very much about dark energy beyond that. Dark energy is also a mysterious force like Nuclear forces. 

So what about the reason of collision ,we can say that between Milky way and Andromeda gravitational force wins over dark energy. This is the reason of collision which is 4.5 billion years away

Depecting the collision by image :

NASA released a computer simulated image to depict the sequence of collision shown below ;

Milky way and Andromeda collision

What will happen after it ?

After the collision both galaxies will combine to form a new galaxy - ( Milkdromeda or Androway ). Many stars systems of both galaxies will destroy.
My opinion

I think mankind will not be alive to see it.

What do you think ?

Comment your answer.