Until the beginning of the last century
astronomers theorized that our galaxy with the billions stars it contains
was the sole Galaxy of the Universe
Practically our Galaxy was the Universe
Until at the beginning of 1920
where Edwin Hubble made an observation
that would change everything
Hubble had at his disposal one of the most advanced telescopes of his era
he managed with extremely detailed observations
to calculate the distance of space objects that were broadly known as nebulas
one of them was known as the Andromeda Nebula
By studying stars that were on this object
Hubble realized that it was very far away
to be part of the Milky Way Galaxy
After many other observations like that he came to the conclusion that the Universe
consists of countless Galaxies that contain billions of stars
therefore the Andromeda Cloud was renamed to Andromeda Galaxy
Andromeda's Galaxy is the closest large Galaxy and lies at a distance of two and a half million light years
Hubble had much more to contribute to science
he used an effect known as the "Doppler Effect "
according to the Doppler Effect, when a light source is moving away from us
the light source's color is shifted to the red end of the spectrum
and when it approaches us the light source's color is shifted to the blue end of the spectrum
Thoroughly observing far away Galaxies
Hubble realized that their color was shifted to the red end of the spectrum
Therefore he came to the conclusion that all of the distant galaxies are moving away from us with high velocities
Also, he made a very important observation that is known as Hubble's law
The more distant a galaxy is, the faster it is moving away from us
Therefore Hubble came to the conclusion that the Universe is expanding
It is easy for someone to be tempted and theorize that if everything is moving away from us
then we must be at the center of the Universe
but in reality wherever we where at the universe we would see distant galaxies receding
Why is this happening?
It might be easier to undestand by be thinking of the surface of an expanding balloon
if you are on the surface of the balloon while it's expanding
you would have the perception that all the other points are moving away from where you are
With the exception of a small number of Galaxies that are gravitationally connected to our Galaxy
all other galaxies are moving away from us
And like we said before the farther they are from us the fastest they recede.
But why is this happening ? why does the distance affect the velocity?
The explanation is that the space between the galaxies is expanding
therefore the more space between the two galaxies
the fastest they move away from each other
This can be illustrated by an example
The distances between the galaxies are the ones we see in the picture
where each circle is one galaxy
After a period of time
if the space between the galaxies is increased uniformly
the distance between them will also increase uniformly
Now let's see how an observer would see that shift from point X
From the observer's perspective it looks like all the galaxies have moved away
and the ones that are farthest have gone even farther
Naturally, it doesn't matter which galaxy we pick as the observer
Any point that we use as our reference point will see the farthest galaxies
moving away from the point with greater velocities than the rest
Newest discoveries show us that the expansion of the universe is not occurring at a stable rate but it is accelerating
The bigger the balloon gets the fastest it expands
that led the cosmologists to the idea of Dark Energy
an exotic form of energy that forces the universe to expand at an accelerated rate
Now, knowing that the universe expands
what would we see if we went back in time ?
Instead of seeing galaxies moving away from one another
they would get closer and closer
until eventually all the matter of the universe would be at one particular point
That particular point is called a Singularity
The temperature at that point is humongous and the density is infinite
The whole Universe emerged from that particular point
Therefore that theory was named Big Bang meaning big explosion
but it's not really an explosion but a very rapid expansion
an expansion that started 13.8 billion years ago
That number is also the age of the universe
The big bang theory has been able to make some predictions
at the first 380,000 years of the universe
matter was extremely dense
and free electrons did not allow light to travel
later the free electrons were bound to  atomic nuclei
creating the first atoms
so the trapped light finally escaped
In 1964 we managed to detect that light now known as Cosmic microwave background
so the Big Bang theory was confirmed
The Big Bang theory has accurately predicted with great accuracy
the quantities of some light chemical elements
as well as the morphology of the great structures of the universe such as  galaxies
Also, the age of the oldest stars
doesn't contradict the age of the universe according to the theory
As always in science, the theory can be improved or enriched
and some aspects of it may even change
but to this day it is the most complete theory for the explanation of the evolution of the universe
As a finishing note it would be nice to take a quick look at the future
The most prevalent theory theorizes that the universe will continue expanding with accelerated rates forever
meaning that billion years from now
the farthest Galaxies will be so far away from us that will be receding with velocities greater than the speed of light
meaning that their light won't be able to reach us
If any beings exist in our galaxy at that time in the future
they will come to the conclusion that we arrivated at before 1920:
The Universe is our Galaxy
However, before that an impressive event will occur
The closest (large) galaxy to ours is the Andromeda Galaxy which is  gravittionally bound to ours
meaning that they move closer
4 billion years from now those two galaxies will collide and unite
If life exists at that time at the galaxy the spectacle that they would see at the night sky would be most impressive
This was the Big Bang theory
I hope you got an idea of what it is about and I'll be glad to read
your comments and questions
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And of course we 'll meet again in our next voyage
Farewell!
