Dark Matter

Dark Matter

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 known as "dark matter" and roughly 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, seen affecting 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—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.

Most such models predict that dark matter particles 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 anti-electrons 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 has to be well understood before detections or limits on these particles can be derived.

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 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 transfers energy to the nucleus of an atom in the detector. The LUX-ZEPLIN (LZ) program uses vessels filled with liquid xenon and senses small amounts of scintillation light produced when the nucleus of a xenon atom in the detector is struck.

This visualization is based on a computer simulation of the large-scale structure formation in the Universe. It shows a region about 400 million lightyears across, starting shortly after the Big Bang when the dark matter was distributed almost uniformly throughout space. Tiny density fluctuations were amplified due to gravity, giving rise to a filamentary structure, called the cosmic web, which is depicted in black in this color scheme. Due to their gravitational pull, the densest regions of dark matter, so-called halos, color-coded in yellow/orange, attract baryonic matter, mainly hydrogen, which collapses and forms stars and galaxies. (The underlying computer simulation did not model baryonic matter, star or galaxy formation processes.)

Dark Matter

Gravitational Lensing

This movie shows a large cluster of several hundred galaxies in the foreground. Most of the mass of the cluster is contributed by so-called dark matter, probably a new type of subatomic particles that do not interact with light and therefore is invisible, but has mass and can be observed indirectly by its gravitational effects on visible matter.

According to Einstein's theory of gravity, the cluster's huge mass substantially warps the nearby space-time structure, causing a deflection of light particles the pass through the region. This results in a distortion of the images of background galaxies, visible as arc-like structures around the cluster - an effect that is called gravitational lensing.

In the first part of the movie, the camera rotates around the center of the cluster. Next, we see the orbital motion of the individual galaxies around the center of mass over time-scales of several hundred million years. The last part shows the hypothetical effects of stripping the invisible dark matter off the cluster: without the gravitational force contributed by the dark matter, the orbits of the galaxies are not stable anymore and the cluster dissolves. At the same time, the gravitational lensing almost vanishes.

Video Gallery

Discovery Retreats: Dr. Risa Wechsler on "What is Dark Matter?"

Public Lecture | A Sparkle in the Dark: The Outlandish Quest for Dark Matter

Searching for Dark Matter, the LUX and LZ Experiments - Dan Akerib