Dark matter constitutes roughly 23-30% of the energy of the universe, yet we cannot directly observe it. Dark matter is a generic term for non-visible matter that we nevertheless know to exist, due to its crucial role in the formation of galaxies, and in preventing galaxies from disassembling. The effect of dark matter is also seen in gravitational lensing, which would be far less intense if not for extra, unobservable matter.
While dark objects causing gravitational effects had been suggested before, the first person to provide an estimate for the amount of dark matter in the Milky Way was Lord Kelvin in 1884. Lord Kelvin estimated that roughly nine-tenths of matter in the galaxy was composed of stars and other celestial bodies not bright enough to be observable from Earth. In 1922, shortly before his death, the influential Dutch astronomer Jacobus Kapteyn published his theory of the arrangement of the galaxy, describing it, correctly, as a flattened disc of stars and gas rotating around a central galactic pole. In his theory, he addressed the concept of dark matter as a possibility, stating that with his theory, the mass of dark matter in the universe could be calculated, lending credence to the theory.
The idea of dark matter was reinforced by Fritz Zwicky, working at the California Institute of Technology. Zwicky calculated that the stars in the Coma Cluster (a galaxy cluster) had only 1% of the mass needed to keep it intact. As a result of this, Zwicky postulated that the cluster was mostly composed of unseen matter, allowing it to remain intact. Zwicky named this matter "dunkle Materie"(Dark matter).
Gravitational lensing is a result of the warping of spacetime, where very massive objects (such as galaxies and black holes) severely distort the space around them, as described in general relativity. This causes light to take what appears to be a curved trajectory, visible to telescopes. The Hubble telescope can observe the lensing effect that galaxy clusters have on the light of more distant galaxies, and use this to calculate their mass. However, this method indicates that dark matter outnumbers normal matter by roughly five to one, by calculating the amount of visible matter and the extent to which light bends.
This method also allows scientists to find where dark matter is concentrated, which would otherwise be impossible, as dark matter cannot be directly observed. They found that a vast network of dark matter filaments are present throughout the universe, with galaxy clusters located at their intersections, where the density of matter is high enough to cause galaxies to form.
One of the potential candidates for dark matter are WIMPs (Weakly Interacting Massive Particles), which are possibly not part of the standard model, and interact with gravity and potentially other forces, though no clear definition of a WIMP exists. The size of potential WIMP particles has been estimated to be roughly 100GeV(gigaelectronvolts), and it is predicted to interact by the electroweak force (a unified field theory for electromagnetism and the weak nuclear force). This conveniently matches a predicted particle in the supersymmetric extension of the standard model, in what is termed the "WIMP miracle".
Supersymmetry is a theory which states that a corresponding particle exists for every particle in the standard model, a "partner particle". This theory would then solve the problem of the mass of the Higgs boson. In the standard model, particles have mass by interactions with the Higgs field and with Higgs bosons. Due to this, the mass of the Higgs boson should be very high, as it interacts with the Higgs field very often. However, the Higgs boson has a much lower mass than this would suggest, creating a problem. This would be fixed by supersymmetry, where interactions with the theoretical "partner" particles would counteract the effect of normal particles, leading to a light Higgs boson.
Based on supersymmetric extensions of the standard model, the lightest supersymmetric particle would have no electrical charge, and would be stable, only weakly interacting with regular particles by gravity. This neatly fits the theory of dark matter, as such a particle would be hard to observe and wouldn't decay, allowing it to survive to today. However, no evidence has been produced proving the validity of supersymmetry, despite extensive testing, and therefore the future of the theory of supersymmetry, and of WIMPs, is in doubt.
In contrast to WIMPs, MACHOs (Massive Astrophysical Compact Halo Objects) do not require a massive change in the understanding of particle physics to exist. MACHO is a broad term used to refer to celestial bodies which emit little or no radiation, making them undetectable by telescopes, and drift through space not as part of any planetary system. Examples of potential MACHOs include black holes, brown dwarfs (a form of "failed star", where their mass exceeds that of the largest gas giants, but they lack enough mass to sustain the fusion of hydrogen-1 in their cores, therefore giving them a very low luminosity), red dwarfs, white dwarfs, neutron stars, and rogue planets (planets not part of any planetary system).
Between 1994 and 2000, research was carried out on the Large Magellanic Cloud (LMC), which aimed to spot microlensing events, where hard to see MACHOs such as black holes moved in front of or near stars, causing gravitational lensing of the light and causing the star to appear brighter. The researchers found many objects they believed to have a mass of roughly 0.5 times that of the sun, potentially accounting for 20% of all dark matter. However, the objects which are the right size to be responsible for this, red and white dwarfs, are still visible to powerful telescopes such as the Hubble Telescope. Searches using these telescopes have conclusively disproved the assertion that these objects account for a significant fraction of dark matter, making the MACHO theory unviable.
Overall, the problem of dark matter remains unresolved, with the two main theories, that dark matter is composed of WIMPs or of MACHOs, either debunked or lacking evidence in support, and a lack of credible alternatives. Currently, the only point on which a consensus has more or less been reached is that dark matter exists, therefore warranting further research into one of the universe's greatest mysteries.
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