A long time ago,
in a galaxy far, far away...
Oh wait! We won't talk about single
galaxy but for the universe as a whole.
In fact this excessively large universe
as it is today
used to be quite small and consequently
infinitely dense and hot.
In the beginning there was a Big Bang and
since then the universe is expanding
when I say the universe
I don't mean matter but space itself.
So as
space itself expands everything seems to
be moving away from everything
Let's assume that the surface of this
balloon is our universe and we take some
random stars on it, as space expands...
the stars are moving away from each other
technically the stars have not moved
at all instead the balloon has just expanded.
Initially scientists predicted
that the expansion would slow down and
possibly stop because of gravity
acting like a break. They even
proposed a scenario where the universe
would shrink back to a single point.
Think of the universe like throwing a ball in the air.
After it reaches a certain point it should fall
back down due to gravity
However, astronomers observed that a certain
type of supernova always appears more
red than expected and this actually proves
that space is expanding in an
accelerating rate. 
Suppose purple light
coming out of a bright start, if this
star is moving the wayfronts are compressed
in front of the star and spread apart
behind. This way the wavelength of the
light behind the star becomes longer
that is a color shifted to the red.
As light coming from all stars around us is
more to the red than expected
we can only conclude that the universe
is expanding. So universe behaves like a
strange ball which when thrown up, instead
of coming back down, it moves faster and
faster up in the sky.
This is where theoretical physics kicks
in and assumes existence of a
mysterious hidden pressure called, geuss what,
dark energy, that pushes the
universe to expand.
Dark energy is
associated with the vaccum and is
distributed evenly throughout the
universe both in space and in time.
Consequently its effect does not get
weaker a space expands.
Even though it's an extremely dark part
of physics the existence of dark energy
has been confirmed. 
We now know that it
accounts for the 68% of the
universe's mass-energy while the
ordinary mass is just 5%.
And...what is the rest 27%?
Well for years now scientists have been
observing galaxies in clusters, where they
rotate around one another
due to gravity. 
It turns out that their
luminous mass is much less than the
mass derived from their velocities of orbiting.
Which means that what we see, is not what we get.
These observations resulted in
the assumption that another type of
matter exists.
Dark Matter.
Dark matter is nothing like the ordinary
matter we know and experience every day.
It does not interact with the electromagnetic
force, therefore it is not visible.
Except for places with high concentration of
dark matter where light is bended
resulting in a different image of
reality. 
We can verify though its
existence and calculate it through its
gravitational effects since it controls
where ordinary matter piles up.
Dark matter is
actually the reason that starts are kept
together in strucures as galaxies and
they're not scattered around.
Dark matter is made of the dark matter
particle which is the first
representative of a new class of
particles called super-symmetric.
We still
don't know how to straightly detect and
observe them.
In the end we realize we still know
quite a few things about such an
important part of our world.
I hope one day I'll manage to help
shed the light to all those dark corners of
physics.
