Only about five percent of the total matter + energy content of the Universe is familiar to us. The identity of the remaining 95 percent, roughly 1/3 of it dubbed as "dark matter" and roughly 2/3, dubbed as "dark energy" is unknown. Though scientists have not yet detected it directly in laboratories on Earth, dark matter’s existence has been deduced from its gravitational effects on the stars and gasses that make up all of the galaxies known in the Universe.
Dark Matter's Crucial Role
In addition to its physical effects, dark matter is a crucial component of the cosmological theory because of its key role in defining the structure of the universe and in binding all galaxies – even our own Milky Way – together. Modern astrophysics and particle physics theory suggests that dark matter exists in the form of a yet undiscovered elementary particle.
Dark Matter Is Everywhere
Dark matter is pervasive throughout the Universe – so it’s no surprise that dark matter is also prevalent on Earth. Based on observations of the motions of nearby stars, theory predicts that one dark matter particle will inhabit a volume the size of your coffee cup. The direct identification of the nature of dark matter will establish a firm connection between physics on the largest astronomical scales and the smallest scales studied in laboratories on Earth.
To better understand our universe, it is often necessary to estimate the mass of an astrophysical object. Those objects can range in size from the Sun, the solar system, the Milky Way, and even the entire universe.
Researchers use a number of techniques to measure the mass of extremely large objects. One way to estimate an object’s mass is by observing its light output. If the object does not emit its own light, researchers can examine the way in which the light of background sources bends around it. Another technique is to examine the dynamic motion of objects around it.
It was long believed that the estimated masses coming from these techniques would agree with one another. However, over the past 80 years, it has become apparent that for objects at the galaxy scale and larger, the amount of mass contained exceeds the mass of it’s luminous constituents. This additional mass, which cannot be accounted for by luminous matter we know about, has been coined “dark matter”.
The nature of dark matter remains a mystery because, so far, we can’t see it directly but only detect its effects indirectly on the large-scale structure of the universe. The most likely form of dark matter is a new class of elementary particles predicted by the so-called “super-symmetric extensions” to the standard model of particle physics.
Most of such models predict that the dark matter particle can “self-annihilate.” This happens when two dark matter particles collide. When particles strike one another, energy is released in the form of detectable standard model elementary particles such as photons or charged particles such as positrons and electrons. Many dark matter models predict the emission of gamma rays, the highest energy photons, as annihilation products. KIPAC researchers are using gamma-ray data from the Fermi Large Area Telescope (LAT) to search for the annihilation products. In order to search for these products, the astrophysical foreground have to be well understood before detections or limits on these particles can be derived. In the future the ground-based Cherenkov Telescope Array (CTA) will search for dark matter annihilation products at even higher masses.
A second way to look for these dark matter particles is with specialized detectors that are well shielded from conventional sources of radiation, and to look for minute energy transfers that are expected when these particles occasionally strike an atomic nucleus in the detector. KIPAC researchers are attempting to detect dark matter with two major research programs. The Cryogenic Dark Matter Search (CDMS) uses silicon and germanium "solid state" detectors that are cooled close to absolute zero, and are sensitive to very small temperature changes when a dark matter particle transfers energy to a nucleus. The LUX-ZEPLIN (LZ)program uses vessels filled with liquid xenon and senses small amounts of UV "scintillation" light produced when a nucleus is struck.