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What Is the Universe Made Of?
We have observed that only some four percent of the total density of matter
and energy in the universe is made of the matter that we know:
the matter out of which stars and planets and ultimately we ourselves are made.
The vast majority of energy density is made of so-called dark energy.
This accounts for about 70 percent of the universe's energy density.
Yet we do not know what exactly we are dealing with here.
And then a couple percent is made up of neutrinos
or some other particles.
And finally we have some 25 percent
that is composed of so-called dark matter.
The big question is: What could this dark matter be made of?
How have we even come to this conclusion?
Numerous observations have led us here.
One such observation, for example, is that of the rotation curves of galaxies.
We understand the velocity of galaxies as a function of distance.
If there were no dark matter,
then the velocity of galaxie's rotation curves would decline with distance.
Yet the actual measurements show us rotation curves
whose velocities do not decline.
We can infer from this data that some other matter has to exist.
And this is in fact the case:
If you take a look at the total density of matter and energy in our universe,
then you'll see that only about four percent of it is our own baryonic matter.
This is the kind of matter that we know, and out of which the planets, stars, and we ourselves are made.
Then there is a small portion of the total density that is made up of neutrinos.
The vast majority -70 percent when all is said and done- is made of dark energy.
And then we have roughly another 25 percent that is made of this mysterious dark matter.
For the vast majority of the universe's density -the dark energy, that is-
we have no explanation at all of what it might actually be.
But what about dark matter?
The Standard Model itself in principle contains a candidate for it,
namely: neutrinos.
It has been discovered that the mass of a neutrino is not equal to zero.
It is very small, but not zero.
However, when we attempt to construct dark matter with this tiny mass,
we find that it can only explain a few percent of the universeís total density,
but by no means the entire 25 percent that is dark matter.
Furthermore, if neutrinos were responsible for dark matter,
this would contradict the cosmological structures of the universe.
Supersymmetric theories -abbreviated as SUSY- provide a very strong candidate.
In these supersymmetric theories,
we have the so-called lightest supersymmetric particle that does not decay any further.
And most of the time this particle is the neutralino.
We indicate this lightest neutralino with the following symbol:
And this LSP (Lightest Supersymmetric Particle) or lightest neutralino
could be responsible for dark matter.
This means that neutrinos alone cannot form dark matter.
Supersymmetric theories, however, have a very good candidate for this dark matter.
In supersymmetric theories, there is a particle,
the lightest supersymmetric particle, which is usually the neutralino.
This particle has the necessary characteristics to form dark matter.
And one of the main goals at the Large Hadron Collider at CERN near Geneva
is to demonstrate supersymmetries.
If they actually succeed in finding supersymmetric particles there,
then we could go and measure the qualities of those particles.
And we could thus find out whether the lightest supersymmetric particles
are actually responsible for the 25 percent of the universe's density that is composed of dark matter.