Only about five percent of the total matter and energy of the Universe is made of the same familiar matter that makes up everything from stars to planets to human beings. The identity of the remaining 95 percent, roughly one-third of which is known as "dark matter" and two-thirds 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, which affect systems on scales from individual galaxies all the way up to the entire observable Universe.
Its prevalence and physical effects have ensured dark matter a crucial place in cosmological theory because of its key role in defining the structure of the Universe and in binding all galaxies—including our own Milky Way—together. Modern astrophysics and particle physics theory suggests that dark matter exists in the form of an as yet undiscovered elementary particle.
Dark matter is pervasive throughout the Universe, so it’s no surprise that dark matter is also prevalent on Earth. Based on the motions of nearby stars, cosmological theory predicts there to be about one dark matter particle per coffee mug-sized volume of space in our immediate vicinity. The direct identification of the dark matter particle will establish a firm connection between physics on the largest astronomical scales and the microscopic 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, to our galaxy, to our 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 its luminous constituents. This additional mass, which cannot be accounted for by the luminous matter we know about, is what has earned the name “dark matter.”
Observing the Effects of Dark Matter
The nature of dark matter remains mysterious because, so far, we can't see it directly; instead, we observe its effects indirectly on the large-scale structure and dynamics of the Universe. New classes of elementary particles are theorized to constitute the dark matter; these range from particles in supersymmetric extensions of the standard model to light “axion” fields thought to arise in string theory.
The properties of the dark matter particle, including its mass and interactions with normal matter, influence how cosmic structure forms and grows. Scientists at KIPAC use observations of galaxies, large-scale structure, and gravitational lensing to test dark matter theories. This research connects the microscopic properties of the dark matter particle to its effects on the largest observable scales in the Universe.
The Search for Dark Matter Particles
Many dark matter models predict that two colliding dark matter particles can self-annihilate, releasing energy in the form of detectable elementary particles such as photons, electrons, or neutrinos. These annihilation products often have large energies, ranging up to the emission of gamma ray photons. KIPAC researchers are using gamma-ray data from the Fermi Large Area Telescope (LAT) to search for dark matter annihilation products. Astrophysical foreground (i.e., sources of gamma-ray emission from known particles) have to be understood precisely to ensure that potential dark matter signals in these data are robust.
A second way to look for these dark matter particles is with specialized detectors that are shielded from conventional sources of radiation, by searching for minute energy transfers that result from dark matter particles striking an atomic nucleus in the detector. KIPAC researchers are attempting to detect dark matter with two major research programs. The Super Cryogenic Dark Matter Search (Super 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 scatters off of an atom in the detector. The LUX-ZEPLIN (LZ) program uses vessels filled with liquid xenon to measure small amounts of UV scintillation light produced when the nucleus of a xenon atom in the detector is struck. KIPAC researchers are also developing new experiments to search for other types of dark matter, including the Dark Matter Radio project, which is designed to detect low-mass axion dark matter particles.