We still don’t know very much about dark matter. The main pieces of knowledge that scientists have are that dark matter has mass and does not emit electromagnetic radiation, which means it cannot be detected by light-collecting telescopes. Its name comes from these two physical properties: it is dark because it does not emit light and it is matter because it has mass. There are two proposed types of dark matter: baryonic and non-baryonic. Baryons are a family of subatomic particles that includes protons and neutrons, which means that all matter composed of atoms is baryonic. If you would like to simplify, think of baryonic dark matter as being composed of atoms and of non-baryonic dark matter as being composed of other kinds of particles.

The most researched candidates for baryonic and non-baryonic dark matter are known, respectively, as MACHOs and WIMPs. MACHOs (massive astrophysical compact halo objects) are celestial bodies that emit little to no radiation, such as black holes and neutron stars. While there are certainly untold numbers of MACHOs that scientists have not observed, recent studies have shown that they are not nearly abundant enough to be the sole source of dark matter in the Universe. WIMPs (weakly interacting massive particles) are hypothetical particles that interact very weakly or not at all with baryonic matter and non-gravitational forces, making them very difficult to detect. While they are called “massive particles,” this refers to their property of having mass, rather than their size, which is smaller than an atom.

There are three main theorized types of non-baryonic dark matter, corresponding with the speeds at which these hypothetical particles move: hot dark matter (HDM), which moves near the speed of light; cold dark matter (CDM), which moves well below relativistic speeds; and warm dark matter (WDM), which has some properties of both other types. The next section will discuss recent and ongoing experiments to find dark matter, including the search for these particles.

There are a number of experiments being performed in the attempt to detect dark matter both directly and indirectly. Most direct detection is focused on WIMPs, including the “hot” neutrino and the “cold” axion. Neutrinos, which have been observed by physicists, are very fast-moving particles with close to zero mass that only interact with one of the three fundamental forces other than gravity, the weak nuclear force. Recent experiments have cast doubt on neutrinos as a solution to the problem of dark matter, although hope remains for a hypothetical “sterile neutrino” that only interacts with gravity.

Axions are hypothetical particles that have very low mass and interact weakly with existing particles, especially photons (light). Two experiments searching for axions are the University of Washington’s Axion Dark Matter Experiment (ADMX) and the US Department of Energy’s LUX-ZEPLIN (LZ) Dark Matter Experiment. The ADMX cools a chamber to very low temperatures and creates an extremely strong magnetic field in an attempt to make local axions convert into photons, causing the release of a tiny amount of energy that can be detected by a hypersensitive instrument. The LZ experiment hopes to observe collisions between axions or other WIMPS and cooled liquid xenon, which would release a flash of light, also detected by a hypersensitive instrument.

Observation of gravitational lensing is a method used for indirect detection of dark matter. Gravitational lensing occurs when light is bent by an extremely strong gravitational field, thousands of times stronger than that of our Sun. Predicted by Albert Einstein’s theory of relativity, it can be used to find areas with high concentrations of dark matter when light travelling to Earth is bent more than it should be by a gravitational field’s visible mass, implying the existence of more invisible mass.

There are many concurrent experiments seeking to detect hypothetical varieties of dark matter. It is likely that there will not be a single “Eureka moment” discovery where scientists find the composition of all dark matter in the Universe. However, we are constantly improving and refining our knowledge of dark matter by running experiments that rule out some theories and continue to pursue others. It may be a slow and frustrating journey at times, but we are well on our way to discovering one of the greatest mysteries of the Universe.