In cosmology, dark matter is an unidentified type of matter that accounts for most of the mass in the universe and explains many of its observed structures. Although dark matter has not been directly observed, its existence and properties are inferred from its gravitational effects on visible matter, radiation, and the large-scale structure of the cosmos. Dark matter is thought to be composed of particles that do not emit or absorb light or other electromagnetic radiation, making it invisible to observers.
The Standard Model of particle physics does not identify any candidate for dark matter, so it is usually considered as part of exotic physics. Numerous experiments have attempted to detect dark matter particles but none have succeeded. The leading candidates for dark matter are Weakly Interacting Massive Particles (WIMPs) and axions; both are currently under active investigation by experimental physicists.
History
In 1933, Swiss astrophysicist Fritz Zwicky used apparent magnitudes and redshifts of galaxies in Coma Cluster to estimate that these galaxies contained 400 times more mass than could be seen by their luminosity. He attributed this discrepancy to “dunkle Materie” which he considered responsible for the cluster’s extra mass and strong gravitational pull that held it together. Zwicky’s estimates were later confirmed by measurements made by Vera Rubin and Kent Ford in 1970s who found similar results while studying orbital velocities of stars within spiral galaxies including our own Milky Way galaxy. Rubin et al.’s discoveries provided much stronger evidence for the existence of unseenmatter because they measured Doppler shifts which cannot be accounted for solely by luminous mass alone.
Doppler shift measurement technique pioneered by Vera Rubin (left) led to stronger evidence for unseenmatter in form of dark Matter (right). Image credit: Carnegie Institution for Science/Vera Rubin Observatory/NSF/AURAIf indeed mostof the massin Universe is invisible, then it must interact via gravity only since otherwise we would’ve detectedit already either directlyor indirectly through electromagnetic interactions(which includevisible light). This hypothetical new formof non-luminousmass became known as “darkmatter”. The name was coinedby astronomerFritz Zwickywho first proposedits existencein 1930s when he noticedthat rotationcurvesof spiralgalaxiesdidn’t fall offas expectedbased on theirvisible massesalone(luminousmatter). It tookseveraldecadesfor observationalevidenceto catchup withZwicky’s speculationand finallyconvincemajorityof scientistsabout realityof darkmatter— oneofthe cornerstonesof moderncosmology today . Even though we still don’t know what exactlydarkmatteris made outof , thereare few generalpropertieswe can infer aboutit from observations:
-Darkmattermakes up the vast majority ofthe totalmassinthe Universe(~84% accordingto latest Planck data); -It doesnot emitor absorblight , makingit completelyinvisibleto us; -It interactswithordinarybaryonicmatternot only gravitationallybut also weaklyinteractswithitself ; -It’sdistributedsmoothlyon large scalesand clumps upon smaller onesformingsubstructureswithin largerhalos suchas galacticdiscsand satellitesatellitegalaxies .