Dark matter is the mysterious stuff that fills entire universe but no one has ever seen. It is something that does not interact with light.

History
Throughout history, natural philosophers have speculated about the nature of
matter, and even have contemplated the possibility that there may be forms of
matter that are imperceptible – because they were either too far away, too dim, or
intrinsically invisible. And although many of the earliest scientific inquiries were
less than rigorous, and often inseparable from philosophy and theology, they
reveal to us the longevity of our species’ desire to understand the world and its
contents.
Although many early civilizations imagined their own cosmological systems, it
was arguably the ancient Greeks who were the first to attempt the construction of
such a model based on reason and experience. The atomists, most famously
Leucippus and Democritus who lived in the 5th century BCE, were convinced that
all matter was made of the same fundamental and indivisible building blocks,
called atoms, and that these atoms were infinite in number, as was the
infinite space that contained them. Epicurus (341 BCE – 270 BCE) further
suggested in his “Letter to Herodotus” that an infinite number of other worlds
existed as well,
“some like this world, others unlike it”
. Others speculated about
unobservable matter that might be found within our own Universe. For example,
the Pythagorean Philolaus conjectured the existence of the celestial body
Antichthon, or counter-earth, which revolves on the opposite side of the “central
fire”
with respect to the Earth.
It was arguably Galileo – who himself had his share of trouble with the inquisition.
By pointing his telescope toward the sky, Galileo saw much that had been
previously imperceptible. Among his many other discoveries, he learned that the
faint glow of the Milky Way is produced by a myriad of individual stars, and that at
least four satellites, invisible to the naked eye, are in orbit around Jupiter. Each of
these observations encapsulate two lessons that remain relevant to dark matter today. First, the Universe may contain matter that cannot be perceived by ordinary
means. And second, the introduction of new technology can reveal to us forms of
matter that had previously been invisible.
Around the end of the 19th century, an interesting discussion began to take place
within the astronomical community. As soon as astronomical photography was
invented, scientists started to notice that stars were not distributed evenly on the
sky. Dark regions were observed in dense stellar fields, and the question arose of
whether they were dark because of a paucity of stars, or due to the presence of
absorbing matter along the line-of-sight.
In 1884, Lord Kelvin estimated the number of dark bodies in the Milky Way from
the observed velocity dispersion of the stars orbiting around the centre of the
galaxy. He estimated the mass of the galaxy, which he determined is different
from the mass of visible stars. He concluded "Many of our supposed thousand
million stars, perhaps a great majority of them, may be dark bodies"
. Kelvin also
obtained an upper limit on the density of matter within such a volume, arguing that
larger densities would be in conflict with the observed velocities of stars. Henri
Poincar
´
e was impressed by Lord Kelvin’s idea of applying the “theory of gases” to
the stellar system of the Milky Way. In 1906 he explicitly mentioned “dark matter”
(“mati`
ere obscure” in the original French), and argued that since the velocity
dispersion predicted in Kelvin’s estimate is of the same order of magnitude as that
observed, the amount of dark matter was likely to be less than or similar to that of
visible matter.
In 1933 over 80 years ago when Swiss-American astronomer Fritz Zwicky (1937
at CalTech) studied the movement of individual galaxies within a huge cluster of
galaxies called the Coma Cluster, The cluster is about 300 million light years from
earth and contains thousands of galaxies that orbit its centre He used the motions
of a few dozen galaxies as a tracer of the gravity field that binds the cluster
together. Zwicky observed that galaxies in the Coma cluster were moving so
quickly they should have been flung away into space, he discovered that their
velocity had a shockingly high value. He inferred an enormous mass for the Coma
Cluster, The total mass of the galaxies in the cluster was not enough to explain the
measured velocities. He discovered that the mass of all the stars in the Coma
Cluster of galaxies provided only about 1 percent of the mass needed to keep the
galaxies from escaping the cluster’s gravitational pull.
In addition to the question of whether the dynamics of galaxy clusters required the
presence of dark matter, astronomers around this time (during the time of 1950s)
began to be increasingly willing to contemplate what this dark matter might be
made of. Herbert Rood (later confirmed by Simon White) studied the relaxation
process of galaxy clusters and argued that the mass responsible for their high
mass-to-light ratios must be found within the intergalactic space, and not in the
galaxies themselves. Arno Penzias searched for free hydrogen in the Pegasus I
cluster and set an upper limit of a tenth of its Virial mass. Neville Woolf suggested
in 1967 that the gas could be ionised, and used radio, visible and X-ray
observations to set limits on it. Turnrose and Rood discussed the problems of this
hypothesis, and in 1971 Meekins obtained observational evidence for X-ray
emission that limited the amount of hot intracluster gas to be less than 2% of that
required for gravitational binding.
Around the same time, Dutch astronomer Jan Oort also found evidence
suggesting the existence of unseen mass in the universe. He noticed that stars in
the Milky Way were moving faster than could be accounted for by the visible
matter alone, leading him to hypothesise the presence of invisible mass.
For the Coma cluster, Ve= 977 km/s and R = 3 Mpc
Implies a mass of 3.3 x 1015 Mo
The mass estimate from visual luminosity is about 5 x 1012 Lo , so the
mass-to-light ratio is M / L = 660Mo/Lo. This shows again the magnitude of the dark
matter problem.
Additional galaxy cluster observations show the same lack of visible mass to
explain the galaxies’ velocities.
The reality of this missing mass remained in question for decades, until the late
1960s and early 1970s, American astronomer Vera Rubin and her colleague Kent
Ford studied the rotation curves of galaxies. They worked with a spectrograph to
measure the velocity curve of edge-on spiral galaxies with greater accuracy. They
found that the outer regions of galaxies were rotating at nearly the same speed as
the inner regions, contrary to what would be expected if only visible matter were
present. This result was confirmed in 1978. An influential paper presented Rubin
and Ford's results in 1980. They showed most galaxies must contain about six
times as much dark as visible mass. This indicated the presence of a significant
amount of unseen mass, reinforcing the idea of dark matter. By around 1980 the
apparent need for dark matter was widely recognized as a major unsolved
problem in astronomy. These results were confirmed over subsequent decades.
The mass of the stars visible within a typical galaxy is only about 10 percent of
that required to keep those stars orbiting the galaxy’s centre. In general, the speed
with which stars orbit the centre of their galaxy is independent of their separation
from the centre; indeed, orbital velocity is either constant or increases slightly with
distance rather than dropping off as expected. To account for this, the mass of the
galaxy within the orbit of the stars must increase linearly with the distance of the
stars from the galaxy’s centre. However, no light is seen from this inner
mass—hence the name “dark matter.
”
The rotation curves of galaxies – i.e. the circular velocity profile of the stars and
gas in a galaxy, as a function of their distance from the galactic centre – played a
particularly important role in the discovery of dark matter. Under some reasonable
simplifying assumptions, it is possible to infer the mass distribution of galaxies
from their rotation curves. Historically, it was the observation of approximately
“flat” rotation curves at very large galactocentric distances that did the most to
convince the scientific community that large amounts of dark matter is present in
the outer regions of galaxies.
Particles
Cosmologists studying the evolution of the universe soon realised that if the
distribution of matter started in an almost perfect state of homogeneity with only
tiny fluctuations around a mean constant density, such small perturbations would,
due to the attractive force of gravity, grow over cosmological time and result in the
huge structures (i.e. galaxies and galaxy clusters) that we observe in the universe
today.
Computer simulations were necessary to study the role of dark matter in such a
highly non-linear process. The first such simulations were performed in the 1960s and today cover the dynamic range from cosmological evolution to galaxy
formation. The simulations demonstrated that the presence of a non-relativistic
and almost collisionless fluid, dubbed cold dark matter, is actually necessary to
form the universe as we see it today. Dark matter indeed ensures the required
growth of structures throughout the history of the universe with the 'visible' matter
falling into the potential wells created by collapsed Dark matter structures named
Dark matter halos. If the matter in the universe consisted mainly of ordinary
particles (interacting with each other and with photons, and thereby exerting a
pressure that opposes the growth of perturbations), the gravitationally bound
structures we observe today, from dwarf galaxies to galaxy clusters, could not
have been formed.
Ever since these discoveries, scientists have been trying to piece together the
puzzle using sparse clues.
Over the past few decades, the very meaning of the phrase “dark matter” has
evolved considerably. Today, this phrase is most frequently used as the name – a
proper noun – of whatever particle species accounts for the bulk of our Universe’s
matter density. When a modern paper discusses the distribution of dark matter, or
the impact of dark matter on structure formation, or the prospects for detecting
dark matter with a gamma-ray telescope, the reader does not have to ask
themselves whether the authors might have in mind white dwarfs, neutron stars,
or cold clouds of gas – they don’t. This is in stark contrast to the earlier usage of
the phrase, in which the word “dark” was a mere adjective, and “dark matter”
included all varieties of astrophysical material that happened to be too faint to be
detected with available telescopes.
Scientists have not been able to identify the particles that make up dark matter.
They know dark matter exists and where it is but cannot directly see it.Dark matter
is an elusive substance that doesn't interact with electromagnetic forces, making
it invisible and detectable only through its gravitational effects. While its exact
nature is still unknown, several types of dark matter particles have been proposed.
Since the 1990s, scientists have been building large experiments designed to
catch elusive dark matter particles, but they continue to come up empty-handed.

WIMPs
When one considers the dark matter problem from the perspective of the standard
model of particle physics, the three neutrinos clearly stand out. Unlike all other
known particle species, the neutrinos are stable – or at least very long lived – and
do not experience electromagnetic or strong interactions. These are essential
characteristics for almost any viable dark matter candidate. And although we
know today that dark matter in the form of standard model neutrinos would be
unable to account for our Universe’s observed large scale structure, these
particles provided an important template for the class of hypothetical species that
would later be known as WIMPs – weakly interacting massive particles. In this
way, standard model neutrinos served as an important gateway particle, leading
astrophysicists and particle physicists alike to begin their experimentation with a
variety of other, more viable, particle dark matter candidates. And although the
first scientists to consider the role of neutrinos in cosmology did not have the dark
matter problem in mind – many being unaware that there was any such problem
to solve – their work helped to establish the foundations that the field of particle
dark matter would later be built upon.
The Standard Model of particle physics contains no suitable particle to explain
the full set of inferred Dark matter properties. DM thus might offer us a glimpse of
unknown physics beyond the Standard Model, and at the least provide critical
evidence that our picture of the fundamental particles and their interactions is
incomplete. The class of DM candidates that has attracted the most attention in
recent decades are so-called weakly interacting massive particles or WIMPs.
WIMPs appear to be the perfect candidate: new particles at the weak scale, that is
with a mass about 100 times the mass of a proton (i.e. about 100 GeV , with GeV
corresponding to a proton mass), they would naturally have been produced in the
right abundance in the early universe.
However, WIMPs are not the only candidates. Many other contenders, to various
degrees motivated by unrelated open problems in the Standard Model, span
orders of magnitude in mass, ranging from ultra-light bosons often referred to as
fuzzy Dark matter (mass 10-22 eV) to the WIMPZillas with masses approaching
the Planck scale (masses <~1018 eV). In addition, models in which DM does not consist of particles but of macroscopic objects (such as primordial black holes)
also present viable candidates.
Axions
WIMPs, have not been found in any of the data collected so far, nor have particles
called axions; both WIMPs and axions are hypothetical elementary particles
proposed to solve outstanding theoretical mysteries in the widely accepted model
of particle physics, the Standard Model, which classifies all known elementary
particles and describes three of the four known fundamental forces : The
Electromagnetic, Weak, and Strong interactions, leaving out gravity. Additional
dark matter candidates include particles called sterile neutrinos, along with
primordial black holes. Some theorists have proposed that modifications to our
theories of gravity might explain away dark matter, though this idea is less
favoured.
Among the particle species contained within the standard model, neutrinos are
the only examples that are stable, electrically neutral, and not strongly interacting,
and therefore are the only known particles that were viewed as potentially viable
candidates for dark matter. Physicists’ imagination, however, would not remain
confined to the standard model for long, but instead would turn to the
contemplation of many speculative and yet undiscovered candidates for the dark
matter of our Universe. In particular, beginning in the early 1970s, many physicists
began to consider the possibility that nature may contain a spacetime symmetry
relating fermions to bosons, dubbed “supersymmetry”
. Supersymmetry requires
that for every fermion, a boson must exist with the same quantum numbers, and
vice versa. Supersymmetry, therefore, predicts the existence of several new
electrically neutral and non-strongly interacting particles, including the
superpartners of the neutrinos, photon, Z boson, Higgs boson, and graviton. If any
of these superpartners were stable, they could be cosmologically abundant, and
may have played an important role in the history and evolution of our Universe.
According to the Standard Model of particle physics, when our universe was born,
the meeting of matter and antimatter should have annihilated each other. That
means that nothing -- no Earth, no sun, no galaxies, no humans -- would exist. But we do. The Standard Model of particle physics explains three fundamental forces
in the universe: electromagnetism, the weak force, and the strong force.
Electromagnetism is the force between any particles that have a charge. The weak
force causes neutrons to break down, and the strong force explains why
subatomic particles such as neutrons and protons hold together. But there are a
few contradictions within the Standard Model, one of them being the imbalance
between matter and antimatter. The Standard Model also does not explain the
existence of dark matter, nor does it explain an observed property of neutrons.
To solve the neutron problem, physicists in 1977 proposed a hypothetical particle
called the axion. Five years later, the axion was found capable of solving the
problem of dark matter as well. Now, Co and co-author Keisuke Harigaya, a
researcher at the Institute for Advanced Study, are suggesting that the axion may
explain yet another problem: the imbalance between matter and antimatter. Their
research has been published in Physical Review Letters. The hypothetical particle
axion is infinitesimally light -- at least billions of times lighter than the proton, and
almost does not interact with normal matter. This explains why they have not yet
been detected, even with instrumentation that allows us to detect protons,
neutrons and electrons. The axion was first hypothesised to address a
contradiction called the strong CP problem. As you may have learned in high
school physics, electrons have a negative charge, protons have a positive charge
and neutrons have no charge. However, neutrons are made up of more elementary
particles called quarks, which do have charges. So physicists expect neutrons to
interact with the electric field, Co says. But they don't. If the axion exists, it would
turn off the interaction between the neutrons and the electric field, solving the
strong CP problem.
The axion may also be a good candidate for dark matter, which is used to explain
the rotational speed of galaxies, another contradiction of the Standard Model. If
galaxies rotated at the speed at which they currently rotate, with the amount of
gravity they have based on luminous matter -- matter we can see because it emits
light -- they would fly apart. There's just not enough gravity to hold them together.
Scientists suggest there must be a huge amount of matter -- such as a vast field
of axions -- in galaxies we cannot see that explains galaxies' rotational speed.

Detection
Gravity is the portal that led to the discovery of DM and is still the only force
known to mediate DM interactions. If the DM particle is to complete the Standard
Model of fundamental particles, however, it should have small but non-zero
interactions with it, opening up the possibility for detection via forces other than
gravity. Intrigued by this perspective, a composite scientific community, from
particle physicists to astronomers, set out to study the promising GeV-TeV range
of masses with unprecedented precision. As a result, several experiments during
the past decades have come online that will explore the various types of
interactions between Standard Model particles and the 'dark' sector.
In 1984, an article by Andrzej Drukier and Leo Stodolsky at the Max Planck
Institute in Munich appeared in Physical Review D, discussing techniques that
might be used to detect neutrinos scattering elastically off nuclei. Among other
possibilities, the article proposed the use of a superconducting colloid detector,
consisting of micron-scale superconducting grains maintained at a temperature
just below their superconducting transition. Even a very small quantity of energy
deposited by the recoil of an incident neutrino could cause a superconducting
grain to flip into the normal state, collapsing the magnetic field and producing a
potentially measurable electromagnetic signal. In January 1985, Mark Goodman
and Ed Witten submitted a paper to the same journal, arguing that this technology
could also be used to detect some types of dark matter particles. Although
Drukier and Stodolsky’s original detector concept was never employed at a scale
sensitive to dark matter, the broader notion of experiments capable of detecting
∼1-100 keV nuclear recoils provided a path through which it appeared possible to
test the WIMP hypothesis.
The proton-proton collider LHC at CERN in Geneva is a true achievement of
mankind, with over 20 countries participating in the effort. It was started in 2008,
and by 2012 had already produced the discovery of the Higgs boson, the last
particle of the Standard Model. The hope is that, before it retires, the collider will
offer a glimpse of physics beyond the Standard Model. DM particles could be
produced in collisions between protons at TeV energies. If so, they would promptly
escape the detector and could therefore be searched for as 'missing mass' in
recorded events.

The earth effectively experiences a DM 'wind' as it moves through the DM halo of
the galaxy on its orbit around the Sun and the galaxy centre. Because of this, it is
possible that dark matter particles scatter against nuclei of atoms in our
detectors, and this provides a direct means to search for the signals of the dark
matter particles. This technology has become extremely advanced with the field
dominated by ton-scale liquid gas detectors that have sensitivities several orders
of magnitude higher than early technology.
MACHOs
As the evidence in favour of dark matter in galaxies and galaxy clusters
accumulated, more and more astronomers began to contemplate what might
make up this faint material. To many astronomers and astrophysicists, the most
obvious possibility was that this missing mass might consist of compact objects
that were much less luminous than – but otherwise qualitatively similar to –
ordinary stars. Possibilities for such objects included planets, brown dwarfs, red
dwarfs, white dwarfs, neutron stars, and black holes. Kim Griest would later coin
the term “MACHOs”
– short for massive astrophysical compact halo objects – to
denote this class of dark matter candidates, in response to the leading alternative
of weakly interacting massive particles,
“WIMPs”
. Although there is a consensus
today that MACHOs do not constitute a large fraction of the dark matter, opinions
differ as to which lines of evidence played the most important role in reaching that
conclusion (for an example of some of the very early arguments that had been
made against MACHOs as dark matter. That being said, two lines of investigation
would ultimately prove to be particularly important in resolving this question:
searches for MACHOs using gravitational microlensing surveys, and
determinations of the cosmic baryon density based on measurements of the
primordial light element abundances and of the cosmic microwave background.
Future possibilities and its Existence
Dark matter research could potentially lead to revolutionary propulsion technology,
enabling spacecraft to scoop up dark matter as fuel. This would allow for rapid
acceleration and the possibility of reaching speeds close to the speed of light.
This technology would significantly reduce the need for carrying large amounts of
fuel, making long-distance space travel more feasible. It would also dramatically
reduce the time required to reach destinations like Proxima Centauri. Beyond
propulsion, dark matter research holds the promise of unlocking new technologies
and inventions that are currently beyond our imagination, expanding our
understanding of the universe and our place within it.
We don’t know what dark matter is yet, but we have good reasons to believe that it
is made up of new elementary particles. One of the biggest challenges in science
today is to directly observe these particles for the first time. But dark matter
particles very rarely interact with ordinary matter. Experiments searching for
collisions between dark matter and atoms must be protected from interference
that would otherwise overwhelm this very rare signal. For this reason, detectors
are shielded by huge amounts of rock in deep underground laboratories.
Another major challenge is that we don’t know the mass of the dark matter
particles we are searching for. Most experiments are sensitive to dark matter that
is roughly 10-1000 times heavier than a hydrogen atom. But it’s possible that dark
matter could turn out to be much lighter, which makes detecting it even more
challenging. Imagine the collision of two balls, but one of them is invisible: If a
visible bowling ball were to bump into an invisible one, they would both recoil, and
we could infer the existence of the invisible ball by the movement of the other. But
if an invisible ping-pong ball were to bump into a bowling ball, the heavier ball
would barely move – and we would have no idea that the ping-pong ball even
existed. So too, if a light dark matter particle bumps into a much heavier atom in
our detector, we would not notice that anything had happened.
The quest to uncover the nature of dark matter is one of the greatest challenges
in science today, but the key to finally understanding this mysterious substance
may well lie in the stars.
Or to be precise, one particular type of star – the neutron star.
So far, scientists have been able to infer the existence of dark matter, but not
directly observe it. Actually detecting dark matter particles in experiments on
Earth is a formidable task, because the interactions of dark matter particles with
regular matter are exceedingly rare. To search for these incredibly rare signals, we
need a very large detector – perhaps so big that it is impracticable to build a
detector large enough on Earth. However, Nature provides an alternative option in
the form of neutron stars – an entire neutron star can act as the ultimate dark
matter detector.
Neutron stars are the densest stars known to exist and form when giant stars die
in supernovae explosions. Left behind is a collapsed core, in which gravity presses
matter together so tightly that protons and electrons combine to make neutrons.
With a mass comparable to that of the Sun – compressed into a 10km radius –
one teaspoon of neutron star material has a mass of about a billion tons. These
stars are ‘cosmic laboratories’
, enabling us to study how dark matter behaves
under extreme conditions that cannot be replicated on Earth. Dark matter interacts
only very weakly with ordinary matter. For example, it can pass through a light-year
of lead (about 10 trillion kilometres) without being stopped. Incredibly, however,
neutron stars are so dense that they may be able to trap all dark matter particles
that pass through them.
Theoretically, the dark matter particles would collide with neutrons in the star, lose
energy, and become gravitationally trapped. Over time, dark matter particles would
accumulate in the core of the star. This is expected to heat up old, cold, neutron
stars to a level that may be in reach of future observations. In extreme cases, the
accumulation of dark matter may trigger the collapse of the star to a black hole.
This means that neutron stars may allow us to probe certain types of dark matter
that would be difficult or impossible to observe in experiments on Earth.
On Earth, dark matter experiments look for tiny nuclear-recoil signals, caused by
incredibly rare collisions of slow-moving dark matter particles. In comparison, the
strong gravitational field of a neutron star accelerates dark matter to
quasi-relativistic speeds, resulting in much higher energy collisions.
A critical challenge in using neutron stars to detect dark matter is ensuring that
the calculations scientists use, fully account for the unique environment of the
star. Although the capture of dark matter in neutron stars had been studied for
decades, existing calculations have missed important physical effects.
Another problem for Earth-based detection is that nuclear-recoil experiments are
most sensitive to dark matter particles that have a similar mass to atomic nuclei, making it harder to detect dark matter that might be much lighter or heavier.
However, dark matter particles can theoretically be trapped in stars and planets in
considerable amounts, regardless of how light or heavy they are.
Experiments generally hunt for dark-matter particles in two ways: either through a
direct search in which dark-matter particles bump into target material and scatter
off atomic nuclei, resulting in a measurable nuclear recoil (these experiments are
usually located underground, where there's little background noise), or through an
indirect search for particles that should appear if a dark matter particle annihilates
(these experiments are generally conducted with ground-based or space satellite
telescopes). It is also thought that if dark matter particles can annihilate into
regular (or Standard Model) particles, then the reverse could be true, and that dark
matter particles could be created during high-energy collisions like those at the
Large Hadron Collider.
One of the earth based detection for dark matter situated in italy, The
DAMA/LIBRA experiment is a particle detector experiment designed to detect
dark matter using the direct detection approach, by using a matrix of NaI(Tl)
scintillation detectors to detect dark matter particles in the galactic halo. The
experiment aims to find an annual modulation of the number of detection events,
caused by the variation of the velocity of the detector relative to the dark matter
halo as the Earth orbits the Sun. It is located underground at the Laboratori
Nazionali del Gran Sasso in Italy.
It is a follow-on to the DAMA/NaI experiment which observed an annual
modulation signature over 7 annual cycles (1995-2002). The experiment was first
proposed by Dr. Pierluigi Belli, who is now the research director of the Italian
National Institute of Nuclear Physics. While DAMA/LIBRA has published exciting
results, the validity of those results has been widely disputed.
There are several other experiments that are planned for further research on dark
matter, like The Quantum Enhanced Superfluid Technologies for Dark Matter and
Cosmology (QUEST -DMC) experiment, The Broadband Reflector Experiment for
Axion Detection (BREAD), Cryogenic Rare Event Search with Superconducting
Thermometers, And many more ongoing experiments.

Conclusion
In conclusion, dark matter remains one of the greatest mysteries in modern
science. Despite our extensive understanding of its gravitational effects and its
crucial role in the formation and structure of the universe, we have yet to directly
observe or definitively identify its constituent particles. The search for dark matter
involves a diverse range of methods, from cutting-edge laboratory experiments to
astronomical observations and theoretical explorations. While numerous
candidates have been proposed, including WIMPs, axions, and primordial black
holes, the elusive nature of dark matter means that no conclusive detection has
been made so far. It is a long, exciting, confusing, magical journey for humanity to
really find dark matter. It would be an evolutionary success. Astrophysicists are
researching, experimenting, and wondering all over the world about this
mysterious phenomenon. The journey to solve the dark matter mystery continues
to fascinate and challenge scientists, representing both the complexity and wonder
of our pursuit for knowledge.