What is dark matter?

Evidence for Dark Matter


When Fritz Zwicky first proposed the existence of Dark Matter in 1933, he based his conclusion on the comparison of measured velocities of galaxies in the Coma galaxy cluster with estimates of the cluster mass from the integrated star light of its member galaxies. The galaxies were observed to be moving much too fast to be held together by the gravitation of the visible mass. It took four decades before this proposition was taken seriously, following the detailed study of rotation curves of spiral galaxies. Here again, on tenfold smaller scale, the visible mass falls dramatically short of explaining the high velocities of stars and gas in the outer parts of the disks of spiral galaxies. Similar tests with other galaxy types result in the same general conclusion. Today, we have striking evidence for the existence of Dark Matter from a wide array of astrophysical observations, on scales ranging from dwarf galaxies to the observable universe as a whole.
Moreover, quite different from what Zwicky might have imagined, we have meanwhile learned what the bulk of Dark Matter is not: neither is it some dark but otherwise “regular” matter (e.g., molecular hydrogen), nor is the phenomenon explainable by more exotic forms of “regular” matter, which exist, e.g., in compact stellar remnants  (White Dwarfs, Neutron Stars, stellar Black Holes) or in failed stars (Brown Dwarfs). Dark Matter is not just transparent matter, it is completely different from any matter we know.
Some of the evidences for Dark Matter are:
• The rotation curves of spiral galaxies.
• The dynamics of galaxy clusters.
• The mass of galaxy clusters that act as gravitational lenses is much larger than the visible (in the whole spectral range) amount of stars, gas, and dust.
• A Fourier analysis of the fluctuations in the cosmic microwave background (CMB) measured with WMAP allows to determine fundamental cosmological parameters very precisely.


The Cosmic Recipe

“Regular” matter is described very successfully by the Standard Model of Particle Physics. It consists of quarks, bound in protons and neutrons to form atomic nuclei, and electrons. In addition, there are neutrinos, which have a small but yet unknown mass.  Observations of the Cosmic Microwave Background (CMB), and of rare forms (isotopes) of light elements in the early universe, combined with calculations of the formation of elements following the Big Bang tell us that atoms make up only 4.6% of the energy and mass budget of the universe, and only about 1/6 of the total matter density. In other words, Dark Matter is about five times more abundant than regular (baryonic) matter. The remainder of the budget is the so-called Dark Energy, the energy content of  “empty” space, which causes an acceleration of the expansion of the universe today.

According to the analysis of the WMAP data, our universe is made up of
• 4.6% ordinary (baryonic) matter,
• 23% cold Dark Matter that builds large structures in the Universe
• 72% of Dark Energy, which is responsible for the accelerated expansion of the Universe.
In other words, we only know about 5% of the content of the Universe. Understanding the nature of the two dark components ranks therefore among the top questions in physics today. At Mainz, we are trying to detect Dark Matter directly in the (underground) laboratory.

Figure courtesy of WMAP / NASA

What is Dark Matter?

What do we know about Dark Matter? As we have seen, it is not made up of quarks and electrons, the ingredients of regular matter. Yet, it forms gravitationally bound clumps, and it does so rather quickly, within the first billion years after the Big Bang. Numerical simulations show that the rapid formation of complex structures of matter out of the very smooth universe that we witness through the CMB at an age of 380,000 years, requires Dark Matter to be non-relativistic (“cold”). This leads to the idea of neutral, massive particles that possibly interact only via weak interactions. These weakly interacting massive particles (WIMPs) have not been discovered yet, but many experiments around the globe are trying to find the WIMP either directly (like the XENON experiment), or indirectly by searching for rare WIMP annihilation processes in the galaxy (Fermi/GLAST).
It is also possible that Dark Matter will be created in the high energy collisions of the LHC accelerator at CERN. This idea is in particular supported by supersymmetric extensions of the Standard Model (SUSY) that predict many new particles. The lightest SUSY particle (in most theories the so-called neutralino) should be stable and is therefore a strong candidate to be the Dark Matter particle.