Another approximate dividing line is that warm dark matter became non-relativistic when the universe was approximately 1 year old and 1 millionth of its present size and in the radiation-dominated era photons and neutrinos , with a photon temperature 2.
Standard physical cosmology gives the particle horizon size as 2 ct speed of light multiplied by time in the radiation-dominated era, thus 2 light-years. A region of this size would expand to 2 million light-years today absent structure formation. The actual FSL is approximately 5 times the above length, since it continues to grow slowly as particle velocities decrease inversely with the scale factor after they become non-relativistic.
In this example the FSL would correspond to 10 million light-years, or 3 mega parsecs , today, around the size containing an average large galaxy. Conversely, much lighter particles, such as neutrinos with masses of only a few eV, have FSLs much larger than a protogalaxy, thus qualifying them as hot. Cold dark matter offers the simplest explanation for most cosmological observations. It is dark matter composed of constituents with an FSL much smaller than a protogalaxy.
This is the focus for dark matter research, as hot dark matter does not seem capable of supporting galaxy or galaxy cluster formation, and most particle candidates slowed early. The constituents of cold dark matter are unknown. Studies of Big Bang nucleosynthesis and gravitational lensing convinced most cosmologists       that MACHOs   cannot make up more than a small fraction of dark matter.
Warm dark matter comprises particles with an FSL comparable to the size of a protogalaxy. Predictions based on warm dark matter are similar to those for cold dark matter on large scales, but with less small-scale density perturbations. This reduces the predicted abundance of dwarf galaxies and may lead to lower density of dark matter in the central parts of large galaxies.
Some researchers consider this a better fit to observations. No known particles can be categorized as warm dark matter. A postulated candidate is the sterile neutrino: Some modified gravity theories, such as scalar—tensor—vector gravity , require "warm" dark matter to make their equations work. Hot dark matter consists of particles whose FSL is much larger than the size of a protogalaxy. The neutrino qualifies as such particle. They were discovered independently, long before the hunt for dark matter: Neutrinos interact with normal matter only via gravity and the weak force , making them difficult to detect the weak force only works over a small distance, thus a neutrino triggers a weak force event only if it hits a nucleus head-on.
The three known flavours of neutrinos are the electron , muon , and tau. Their masses are slightly different. Neutrinos oscillate among the flavours as they move. It is hard to determine an exact upper bound on the collective average mass of the three neutrinos or for any of the three individually. CMB data and other methods indicate that their average mass probably does not exceed 0. Thus, observed neutrinos cannot explain dark matter. Because galaxy-size density fluctuations get washed out by free-streaming, hot dark matter implies that the first objects that can form are huge supercluster -size pancakes, which then fragment into galaxies.
Deep-field observations show instead that galaxies formed first, followed by clusters and superclusters as galaxies clump together. If dark matter is made up of sub-atomic particles, then millions, possibly billions, of such particles must pass through every square centimeter of the Earth each second.
Another candidate is heavy hidden sector particles that only interact with ordinary matter via gravity. These experiments can be divided into two classes: Direct detection experiments aim to observe low-energy recoils typically a few keVs of nuclei induced by interactions with particles of dark matter, which in theory are passing through the Earth.
After such a recoil the nucleus will emit energy as, e. To do this effectively it is crucial to maintain a low background, and so such experiments operate deep underground to reduce the interference from cosmic rays. These experiments mostly use either cryogenic or noble liquid detector technologies. Cryogenic detectors operating at temperatures below mK, detect the heat produced when a particle hits an atom in a crystal absorber such as germanium. Noble liquid detectors detect scintillation produced by a particle collision in liquid xenon or argon.
Cryogenic detector experiments include: Both of these techniques focus strongly on their ability to distinguish background particles which predominantly scatter off electrons from dark matter particles that scatter off nuclei. Currently there has been no well-established claim of dark matter detection from a direct detection experiment, leading instead to strong upper limits on the mass and interaction cross section with nucleons of such dark matter particles. This results from the expectation that as the Earth orbits the Sun, the velocity of the detector relative to the dark matter halo will vary by a small amount.
A special case of direct detection experiments covers those with directional sensitivity. This is a search strategy based on the motion of the Solar System around the Galactic Center. WIMPs coming from the direction in which the Sun travels approximately towards Cygnus may then be separated from background, which should be isotropic.
Indirect detection experiments search for the products of the self-annihilation or decay of dark matter particles in outer space. For example, in regions of high dark matter density e. These processes could be detected indirectly through an excess of gamma rays, antiprotons or positrons emanating from high density regions in our galaxy or others. A few of the dark matter particles passing through the Sun or Earth may scatter off atoms and lose energy.
This could produce a distinctive signal in the form of high-energy neutrinos. Many experimental searches have been undertaken to look for such emission from dark matter annihilation or decay, examples of which follow. The Energetic Gamma Ray Experiment Telescope observed more gamma rays in than expected from the Milky Way, but scientists concluded that this was most likely due to incorrect estimation of the telescope's sensitivity.
The Fermi Gamma-ray Space Telescope is searching for similar gamma rays. At higher energies, ground-based gamma-ray telescopes have set limits on the annihilation of dark matter in dwarf spheroidal galaxies  and in clusters of galaxies. They could be from dark matter annihilation or from pulsars.
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No excess antiprotons were observed. In results from the Alpha Magnetic Spectrometer on the International Space Station indicated excess high-energy cosmic rays that could be due to dark matter annihilation. An alternative approach to the detection of dark matter particles in nature is to produce them in a laboratory. Because a dark matter particle should have negligible interactions with normal visible matter, it may be detected indirectly as large amounts of missing energy and momentum that escape the detectors, provided other non-negligible collision products are detected.
Because dark matter remains to be conclusively identified, many other hypotheses have emerged aiming to explain the observational phenomena that dark matter was conceived to explain. The most common method is to modify general relativity. General relativity is well-tested on solar system scales, but its validity on galactic or cosmological scales has not been well proven. A suitable modification to general relativity can conceivably eliminate the need for dark matter. A problem with alternative hypotheses is that the observational evidence for dark matter comes from so many independent approaches see the "observational evidence" section above.
Explaining any individual observation is possible but explaining all of them is very difficult. Nonetheless, there have been some scattered successes for alternative hypotheses, such as a test of gravitational lensing in entropic gravity. The prevailing opinion among most astrophysicists is that while modifications to general relativity can conceivably explain part of the observational evidence, there is probably enough data to conclude there must be some form of dark matter. In philosophy of science , dark matter is an example of an auxiliary hypothesis, an ad hoc postulate that is added to a theory in response to observations that falsify it.
It has been argued that the dark matter hypothesis is a conventionalist hypothesis, that is, a hypothesis that adds no empirical content and hence is unfalsifiable in the sense defined by Karl Popper. Mention of dark matter is made in works of fiction. In such cases, it is usually attributed extraordinary physical or magical properties.
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Such descriptions are often inconsistent with the hypothesized properties of dark matter in physics and cosmology. From Wikipedia, the free encyclopedia. Not to be confused with antimatter , dark energy , dark fluid , or dark flow. For other uses, see Dark Matter disambiguation. Discovery of cosmic microwave background radiation. Religious interpretations of the Big Bang theory. Simulated Large Hadron Collider CMS particle detector data depicting a Higgs boson produced by colliding protons decaying into hadron jets and electrons. String theory Loop quantum gravity Loop quantum cosmology Causal dynamical triangulation Causal fermion systems Causal sets Event symmetry Canonical quantum gravity Superfluid vacuum theory.
This section needs additional citations to secondary or tertiary sources such as review articles, monographs, or textbooks. Please add such references to provide context and establish the relevance of any primary research articles cited. Unsourced material may be challenged and removed. Type Ia supernova and Shape of the universe. What is dark matter?
How is it generated? Is it related to supersymmetry? Not to be confused with Missing baryon problem. White, The evolution of large-scale structure in a universe dominated by cold dark matter.
Dark matter feels fake. MOND sounds plausible. What should you conclude?
Alternatives to general relativity. Dark matter in fiction. See Baryonic dark matter. Strictly speaking, electrons are leptons not baryons ; but since their number is equal to the protons while their mass is far smaller, electrons give a negligible contribution to the average density of baryonic matter.
Baryonic matter excludes other known particles such as photons and neutrinos. Hypothetical primordial black holes are also generally defined as non-baryonic, since they would have formed from radiation, not matter. The Dallas Morning News. Annual Review of Astronomy and Astrophysics. Retrieved 21 March An invisible, essentially collisionless component of matter that makes up about 25 percent of the energy density of the universe Retrieved 10 June Evidence, candidates and constraints".
Towards a new paradigm for structure formation". Monthly Notices of the Royal Astronomical Society. It is incidentally suggested that when the theory is perfected it may be possible to determine the amount of dark matter from its gravitational effect. Three Parts Dark Matter. The New York Times. Retrieved December 27, Retrieved 6 August Observational evidence and detection methods". Reports on Progress in Physics.
It is basically the same except that dark energy might depend on scale factor in some unknown way rather than necessarily being constant. Retrieved 8 December Physics for the 21st Century. Rik Myslewski, The Register. Wilkinson Microwave Anisotropy Probe. National Aeronautics and Space Administration. Retrieved 9 January See Wayne Hu The Astrophysical Journal Supplement. Explicit use of et al. Low 12 October Modern Physics Letters A. The Astrophysical Journal Letters. Did the Bullet Cluster withstand scrutiny?
Retrieved 16 March Retrieved 9 February Swinburne University of Technology. Retrieved 9 April Cooking up the first light elements " in: An Introduction to the Science of Cosmology.
There’s A Debate Raging Over Whether Dark Matter Is Real, But One Side Is Cheating
Archived from the original on 30 May Limits on Baryonic Matter in the Galactic Halo". If it could, then it might bunch together and create compact objects in the same way that baryonic matter forms planets, stars, and galaxies. Observations so far suggest that dark matter doesn't do that—it resides only in diffuse halos As a result, it is extremely unlikely there are very dense objects like stars made out of entirely or even mostly dark matter.
Henry Holt and Company. MACHOs can only account for a very small percentage of the nonluminous mass in our galaxy, revealing that most dark matter cannot be strongly concentrated or exist in the form of baryonic astrophysical objects.
Although microlensing surveys rule out baryonic objects like brown dwarfs, black holes, and neutron stars in our galactic halo, can other forms of baryonic matter make up the bulk of dark matter? The answer, surprisingly, is no Retrieved 6 January Annual Review of Nuclear and Particle Science. Retrieved 26 December Dark-Matter Wind Sways through the Seasons".
Peter, and Benjamin R. Safdi 3 January Retrieved 7 January Scientists at Kavli MIT are working on Retrieved 16 June Observations, Models and Searches. Retrieved 20 June While their existence has not been established with certainty, primordial black holes have in the past been suggested as a possible solution to the dark matter mystery. The LIGO findings, however, raise the prospect anew, especially as the objects detected in that experiment conform to the mass predicted for dark matter.
Predictions made by scientists in the past held that conditions at the birth of the universe would produce lots of these primordial black holes distributed approximately evenly in the universe, clustering in halos around galaxies. Those things include the motions of stars and galaxies through the universe, and the bending of light around objects with mass.
We know it's not like other matter that we see in the universe, but that's pretty much all we've got. If it exists, it helps explain a lot—and makes up the vast majority of the universe's total mass. The dark matter is really the skeleton on which everything is built. Having identified the blobby galaxy with Dragonfly, van Dokkum and colleagues started comparing it to other maps of the sky.
They noticed about 10 bright points of light within the galaxy; brilliant bursts of stars known as globular clusters. Basically, the more mass there is, the faster things move. So by measuring our planet's movement around the sun—even if you didn't actually know the sun existed—you could determine the star's mass.
This Is How Much Dark Matter Passes Through Your Body Every Second
He does the same thing with galaxies. By measuring the motion of stars or clusters inside the galaxy, scientists can figure out how much mass it has. And this galaxy is moving very, very slowly. So slowly, in fact, that clusters almost appeared immobile. It's also odd in how it compares to others that seem superficially similar. A few years ago van Dokkum and colleagues identified a galaxy made up almost entirely of dark matter. This is the exact opposite, but both giant galactic blobs appear pretty similar.
Typically, astronomers assume that by looking at the mass of stars in a galaxy they can get an accurate estimate of dark matter within. But if you go down to these very diffuse objects, that apparently completely breaks down.
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Some alternative strategies include re-examining our basic understanding of gravity and rejecting the idea of dark matter, saying that instead all the movements of the stars and galaxies can be understood with a different interpretation of physics. But van Dokkum says this object poses a real problem to those theories. If the modified gravity ideas hold true, and dark matter doesn't exist, then the laws of physics that govern all galaxies should be the same; the physical structures that explain away dark matter's influence in one galaxy should affect this one, too.
So under that paradigm shift, this new galaxy would appear to defy the laws of physics.