The quest for dark matter

It sounds unbelievable, but everything that astronomers can see with their telescopes - stars, nebulas, dust clouds and planets - doesn't even make up five percent of the universe. The rest is mostly invisible and can only be recognized by its gravitational pull.

One example of this is so-called dark energy. It makes up almost 70 percent of the cosmos, as new measurements of European satellite PLANCK have shown. Dark energy seems to be evenly distributed throughout the universe and acts as a repellent, pushing everything farther apart and making space expand faster and faster.

The rest of the universe, roughly 25 percent, is made up of dark matter, which, with its gravitational pull, holds galaxies and galaxy clusters together. Just like foam gathers on the surface of water, visible matter is concentrated in places where dark matter is the densest.

Evidence of dark matter

Scientists reallized back in the 1930s that this mysterious matter must exist. Swiss physicist and astronomer Fritz Zwicky discovered that the visible matter of the Coma Cluster wasn't enough by far to hold the cluster of more than 1,000 individual galaxies together.

Today, with special programs like the Millennium Simulation, astrophysicists let cosmic structures grow virtually and simulate the development of outer space on their computers.

The computer becomes a time machine; eternities shrink to a couple of seconds, so the scientists can witness processes which took place across millions, or even billions, of years in nature. One of these processes is the birth of so-called filaments, expansive cosmic structures consisting of many galaxies or galaxy clusters. In the simulation, they only take a few minutes to form.

With the cosmic creation experiments, researchers can continuously change the composition of their virtual universes. They have found, however, that their virtual cosmos only resembles reality if it has just as much dark matter as is present in the real universe.

Larger, faster, more

Dark matter is a subject of keen interest for researchers working with the Large Hadron Collider (LHC), the huge particle accelerator at the European Organization for Nuclear Research, CERN, in Geneva, Switzerland. In the coming years, physicists there want to delve into energy realms which have never before been produced on earth. For this, they will take larger proton packages, accelerate them more and have them collide more often than before.

An even hotter ball of fire will spring from these collisions in the center of the huge detectors. This fire ball will spark tiny particles: the building blocks of matter. The higher the energy, the heavier the new particles can be.

But the scientists also hope to see a very lightweight particle: the long searched-for component of dark matter, which is produced when dark matter decays.

Researchers at the Max-Planck-Institute for Physics in Munich are already putting together virtual simulations of how to find dark matter particles with the LHC detector at CERN. "These simulations help us find out what to look for in the huge data stream later," Hubert Kroha from the Max-Planck-Institute says.

Doorway to the unknown: Higgs-Boson

An important role in all this is ascribed to the Higgs boson, which was recently discovered at CERN. For scientists, this particle could be the decisive factor in getting their hands on dark matter because the Higgs boson is responsible for the mass of all elementary particles.

What's more, dark matter mostly acts through gravitational pull. "The Higgs boson could interact with dark matter," Kroha said. That would yield valuable information on the elusive matter's properties - the reason why scientists speak of the "Higgs Portal to Dark Matter."

Detecting the invisible

Dark matter itself won't be visible in the detectors. The physicists will only recognize it by its "missing" energy: dark matter doesn't leave traces like the other matter building blocks that are created when protons collide.

Scientists know exactly how much energy is produced when two protons collide. They can measure the energy from the emerging matter components precisely and trace their trajectory through the layers of the huge ATLAS detector. In most collisions, the building blocks of matter scatter evenly in all directions. And the components add up to just as much energy as was produced when the protons collided.

If there's an imbalance of measured energy, however, and the visible particles have a lot less energy than was produced in the collision, that could point to the presence of dark matter. With the help of models that describe all sorts of possible reactions that particles might show, physicists can find out what dark matter is made of.

What is dark matter made of?

Should the scientists at CERN succeed in proving that dark matter is made up of particles, and if they can then decode these particles, they would be a hugely significant step closer to a long-cherished dream: the "world formula."

This is a theory that unites all of nature's four fundamental forces: gravity, the strong interaction that holds an atom's nucleus together, the weak nuclear force that is a precondition for radioactivity, and electro-magnetism.

This world formula - which unites microcosm, macrocosm, quantum mechanics and relativity theory - could open up new insights into the birth of the universe - and the origins of our existence.