In order to understand what dark matter is, we must first define what it isn’t.
Dark matter is not the same as dark energy. As previously discussed, dark energy is an elusive and inexplicable repulsive force acting on the entirety of space to accelerate its expansion. It has no mass, and interacts with no other forces in the known universe. Dark matter on the other hand has gravitational properties, thus it attracts, rather than repels.
Dark matter also is not ordinary matter. Ordinary matter, the stuff that makes up stars and planets and people, is the type of matter that interacts with the all of the fundamental forces of nature. Because of it’s interaction with the electromagnetic spectrum, we can see light it produces or reflects. Dark matter does not interact with or produce electromagnetic radiation, so we cannot detect it directly. It also does not physically interact with ordinary matter, which is why we can’t “feel” dark matter, even though it is all around us.
So How do we know it exists?
If we can’t see or feel dark matter, why do we think such a thing exists? The answer lies in the one thing dark matter does interact with – gravity.
In the early 1930’s astronomers started calculating the orbital speeds of stars in the milky way galaxy. If only ordinary matter existed in the galaxy, one would’ve expected the center to be spinning significantly faster than the outer edges, much like water down a drain. However they discovered the stars on the outer edge were rotating with similar orbital speeds to the center – almost like a Frisbee. To account for this strange behavior, the idea of dark matter was proposed.
Dark matter is thought to be some type of yet-undiscovered particle that interacts heavily with gravitational fields, like ordinary matter. However because it cannot interact with light, we have no way of observing it directly. Thus, we can only determine its existence based on its gravitational effects on visible ordinary matter. It is believed there is a massive amount of dark matter permeating the Milky Way galaxy, and all galaxies, which accounts for the additional rotational velocities observed. Dark matter is thought to account for approximately 84.5% of the total matter in the universe.
WIMPs and Experimental Detection
The prevailing theory of dark matter is the concept of WIMPs – Weakly Interacting Massive Particles. These particles are thought to interact with ordinary matter only through gravity and the weak nuclear force. Scientists are attempting to detect WIMPs both directly and indirectly. If they do in fact interact with ordinary matter via the weak force, there is a chance a collision with the nucleus of an atom could be detected directly. There are also numerous theories for how WIMPs could be detected indirectly from high mass celestial objects such as galaxy centers and the sun. Indirect detection involves looking for excessively high gamma rays or neutrinos that cannot be explained with the standard model of particle physics.
CDMS
One of the primary direct detection methods of WIMPs is known as CDMS (Cryogenic Dark Matter Search) uses hockey puck sized detectors of germanium and silicon crystals. These detectors are kept far underground to shield them from other sources of subatomic particle and radiation interactions, and are supercooled to just a fraction of a degree above absolute zero. The most sensitive detector is known as SuperCDMS, located in the Soudan Mine in Minnesota. Recently the DOE Office of High Energy Physics and the NSF have announced funding for a new CDMS project, known as SuperCDMS SNOLAB, which will be completed in 2018 at an underground lab in Ontario, Canada. SuperCDMS SNOLAB will be nearly three times as deep as the Soudan lab, making it much more sensitive to WIMP interactions.
LUX
Another direct detection experiment is known as the Large Underground Xenon experiment (LUX), located at the Sanford Underground Laboratory in Lead, South Dakota. The LUX Operates similarly to CDMS, but instead of using supercooled crystals, it uses liquid Xenon. The principal is that if a WIMP interacts with the nucleus of the Xenon atom, it will produce both a photon of light as well as an escaping electron, which can be detected.
As of now, scientists are still hunting for a definite WIMP detection, as all of the possible detections have yet to be confirmed. In the coming years as detectors get increasingly sensitive, we will have a better picture of whether or not the WIMP model of dark matter is accurate.