Galaxies within the universe have been observed to be rotating much faster than we had initially predicted. The gravity produced by the matter which we can observe is not enough for these speeds to be reached, therefore scientists believe that there is extra mass in the universe that we just cannot see, dark matter.
Normal matter, called baryonic matter, is made up of quarks and leptons. These particles are detectable due to their interaction with the electromagnetic force, absorbing and releasing electromagnetic waves. Most scientists believe that dark matter is not baryonic, meaning it will not interact with the electromagnetic force but will interact with the gravitational force. The current candidates for dark matter include WIMPs and MACHOs:
Weakly interacting massive particles (WIMPs)
These hypothetical particles don’t interact with the electromagnetic or strong force. They are thought to only interact with other particles via the gravitational and weak forces. Neutrinos, axions, and neutralinos are thought to be potential candidates for this type of particle.
Although neutrinos are part of the standard model, they are very difficult to detect and were initially thought to have no mass. However, neutrinos with mass have since been discovered so they could be a potential candidate for dark matter.
Axions are hypothetical particles with very low mass, which were first suggested to solve problems within quantum chromodynamics.
Neutralinos are theoretical super symmetrical particles. These particles are partners of one elementary particle, with all the same quantum numbers except spin, which differs by 1/2. Neutralinos are thought to have a mass much greater than the proton and scientists are currently attempting to find a way to detect them.
Massive astronomical compact halo objects (MACHOs)
These objects are made up of normal, baryonic matter but are very hard to detect due to the fact they emit very little light. Examples of these include brown dwarfs and neutron stars. However, it is unlikely there are enough undetected to account for all the dark matter in the universe.