The universe... surprise! is full of things.
Things that resist being seen at least through a simple telescope.
The cosmos not only emits visible light
it also emits infrared, ultraviolet, microwave, gamma rays
we receive electromagnetic radiation from all corners of the universe.
and if you know how to sense it, you can
see what's hiding within
but as if this weren't enough, there are still plenty of things out there that we still can't quite observe
so how do we know they
exist in the first place?
well,  because as invisible as they seem to be, gravity gives them away
you only need to see how the gas and stars from the outermost parts of the galaxy
rotate around the center of the galaxy
Newton's gravitational law tells us that their rotational velocity
should diminish the further away from the galactic nucleus they are
but that isn't what we see. The
speed of rotation does not appear to change
Some scientists thought that perhaps the way we understood gravity was flawed
and that we had to adjust it.
Others thought that perhaps the galaxies aren't quite as small as they appear to be
but rather were surrounded by an immense halo that is nine times more massive than they are,
an invisible prolongation whose enormous mass
would change the manner in which objects rotate in the galaxy
This idea gathered momentum in light of further evidence:
unexplained galactic movements,
gravitational lenses without visible matter, ...
they have even found their footprint in the Cosmic Microwave Background.
And this all points towards the existence of something that fills the universe
that exerts a gravitational influence on its surroundings but that we aren't able to see
and this something is called dark matter.
Ah! and beware!
it's important not to confuse dark matter
with either dark energy or with antimatter,
these are three completely
different things
Ok! so the next question is: what is dark matter?
let's see what we know about it:
we know that it is electrically neutral and dark,
i.e. it doesn't interact with  electromagnetic waves, as it neither absorbs and emits them
we know that it is stable, i.e. it doesn't decay over time,
and that it is cold, as it doesn't move at speeds close to that of light
So, who are the suspects then?
Let's ask the Standard Model, the theory which presents us
with the list of the most elementary particles of the universe
A pity that they can't help us!
neither ordinary matter nor neutrinos,
the main candidates, fit the profile
This is a sign that in order to understand
dark matter, we need some kind of new physics,
the physics that goes beyond the
standard model
This is where exotic new candidates enter the scene,
such as axions, supersymmetric particles, extra-dimensional overlords,...
there are quite a few aspirants to Dark Matter.
Without a doubt the real challenge is to
detect it experimentally
which is quite a hard task,  given that not only does dark matter not irradiate anything,
it also barely interacts with matter.
Remember that our planet is within that huge halo,
we're constantly being traversed by particles of dark matter and our machines don't seem to notice them
In spite of that, physicists have a couple of methods for detecting them:
On the one hand there are what are known as directed detection methods;
these attempt to observe collisions between dark matter particles and atoms from a detector.
For instance, in the case of a crystal, this collision would produce vibrations of the detectors lattice
so that it's ionization and small light
emission could be measured.
These experiments are not easy to carry out
as they need to be shielded and isolated underground
in order to dismiss any potential collisions from other particles.
On the other hand there are
also indirect detection methods;
these find the products of possible interactions
with dark matter particles in the halo of our galaxy:
neutrinos, antimatter, even photons,  which we can capture with a variety of experiments
from gigantic neutrino telescopes at the bottom of the Mediterranean and the Antarctic ice
to detectors mounted on satellites as well as gamma-ray
telescopes on the Earth's surface.
It's pretty clear that the entire planet
is on the hunt for dark matter,
and so are we!
Multidark is a project led by Spanish investigators
and its objective is to identify and detect dark matter
supporting theoretical investigation, as
well as experiments being carried out around the globe
so thanks for your work, Multidark!
and I hope that soon dark matter will join that part of the
universe that we can't see but we can understand
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