Dark matter is a mysterious form of matter that does not interact with light or other forms of electromagnetic radiation, making it invisible to direct detection. While its exact nature is still unknown, several candidates have been proposed to explain the gravitational effects attributed to dark matter. Here are a few of the prominent candidates:
Weakly Interacting Massive Particles (WIMPs): WIMPs are hypothetical particles that interact weakly with ordinary matter and have a mass in the range of several times that of a proton. They are popular candidates due to their potential for thermal production in the early universe, which could explain the observed abundance of dark matter. Examples of WIMP candidates include the neutralino (a supersymmetric particle) and the axion (a hypothetical low-mass particle).
Axions: Axions are hypothetical particles initially proposed to solve a problem in particle physics known as the strong CP problem. They are extremely light and have very weak interactions with matter. Axions have gained attention as potential dark matter candidates due to their properties and the fact that they could be produced abundantly in the early universe.
Sterile Neutrinos: Neutrinos are known to be very light and weakly interacting, but sterile neutrinos are hypothetical heavier counterparts that do not participate in the weak nuclear interaction. Sterile neutrinos have been suggested as dark matter candidates, as they could provide the right properties to account for the observed gravitational effects. Several experiments have been conducted to search for their existence.
Primordial Black Holes: Primordial black holes are hypothetical black holes that could have formed in the early universe shortly after the Big Bang. Unlike black holes formed from stellar collapse, primordial black holes could have a wide range of masses, including some in the mass range of dark matter. They are an intriguing candidate as their gravitational effects could explain some dark matter observations.
Modified Gravity Theories: Instead of positing the existence of new particles, modified gravity theories propose modifications to the laws of gravity on cosmological scales. These theories seek to explain the observed gravitational effects without invoking the presence of dark matter particles. Examples include Modified Newtonian Dynamics (MOND) and Modified Gravity (MOG).
It's important to note that while these candidates have been proposed, none of them have been conclusively proven to be the source of dark matter. The search for dark matter is an active field of research, and experiments continue to explore various detection strategies and investigate the properties and existence of these and other potential candidates.