Searching for Dark Matter: Evidence from WIMPs, Axions, and

Primordial Black Holes

Tianjiao Kang

Abbey College Cambridge, Cambridge, U.K.

Keywords: Axions, Dark Matter, Primordial Black Holes, Standard Model, WIMPs.

Abstract: As a matter of fact, dark matter remains one of the most profound mysteries in modern astrophysics and

cosmology. Though it does not emit, absorb, or reflect light, its presence is inferred from its gravitational

effects on visible matter, radiation, and the large-scale structure of the universe. This study explores the

historical development of dark matter thesis, the various lines of observational evidence supporting its

existence, e.g., galactic rotation curves, gravitational lensing, and cosmic microwave background radiation

and the leading theoretical models proposed to explain its nature. Particular attention is given to WIMPs,

Axions, and Primordial black holes, the top three practical candidates of dark matter. Additionally, the paper

reviews current and future experimental efforts aimed at direct and indirect detection of dark matter. By

synthesising contemporary findings, this work aims to provide a comprehensive overview of the dark matter

problem and highlight the challenges and prospects in uncovering its true identity.

1 INTRODUCTION

The first thing that may spring to mind while

discussing dark matter is that it is “dark”. The idea of

dark matter emerged in the early 20th century when

astronomers were attempting to figure out the

universe’s total mass density from stellar masses in

galaxies. It became evident that non-stellar baryonic

matter existed in a gaseous phase within the vacuum

of space (Gordon, 2020; Strumia, 2023). The

abundance of deuterium and other light elements in

primordial gas clouds suggested that the density

parameter for baryonic matter was much lower than

the value required by cosmic observations, implying

that a significant portion of mass must consist of non-

baryonic matter, namely dark matter (Gordon, 2020).

In 1930, Swiss American astronomer Fritz

Zwicky made the prediction that the Coma Cluster's

galaxies would be destroyed because the cluster's

galaxies' radial velocities were dispersed at a rate of

about 1000 km/s (Dutta, 2018; CERN, 2021). The

stars and gas clouds did not provide enough

gravitational attraction to hold the cluster together.

The same issue was faced by American astronomer

Vera Rubin later in the 1970s: stars at the galaxy's

margins were travelling too quickly to be embedded

in the luminous matter of the galaxy; therefore, there

must be another force in the universe that is invisible

to the naked eye that keeps the stars in orbit (Gordon,

2020). According to the most recent observation, dark

matter has an abundance of 26.4% of the whole

universe (Workman et al., 2024).

The dark matter field has been continuously

explored over the last ten years as its discovery will

solve the most significant physics puzzles and

advance the attempts to unify the laws of physics

(Harigaya & Lou, 2023). Following the discovery of

the Higgs Boson in 2012, scholars have been

focussing more on the quantum side of physics,

employing known particles from the Standard Model

(CERN, 2022; Luo, 2024). Dark matter is non-

interactive with ordinary baryonic matter and

radiation, but it does exhibit minimal interactions

with gravity, hence, dark matter’s gravitational pull-

on galaxies is one of the traditional methods of

detecting it.

Researchers are now delving further into the

micro-scale thanks to the 2012 discovery of the Higgs

Boson, which had a major influence on particle

physics and helped uncover other dark matter

candidates such as sterile neutrinos (Ahmed, et al.,

2022). The primordial black holes, for instance, are

one theory on dark matter (Workman et al., 2024).

Furthermore, scientists are now examining the anti-

matter side of the search for dark matter after the LHC

252

Kang, T.

Searching for Dark Matter: Evidence from WIMPs, Axions, and Primordial Black Holes.

DOI: 10.5220/0013823100004708

Paper published under CC license (CC BY-NC-ND 4.0)

In Proceedings of the 2nd International Conference on Innovations in Applied Mathematics, Physics, and Astronomy (IAMPA 2025), pages 252-257

ISBN: 978-989-758-774-0

at CERN revealed the presence of anti-matter. On the

other hand, hypothesis is also put out to refute the

presence of dark matter by modifying the gravity

models that allows the anomalous measurements of

galaxies.

Dark matter's considerable influence on the

composition and development of the universe serves

as the driving force behind research into it (CERN,

2022). Observable and detected evidence, such as

gravitational lensing, galaxy rotation curves, and

fluctuations in the cosmic microwave background, all

points to the presence of a form of non-baryonic

matter that does not interact with light (Irastorza,

2021). Dark matter contributes significantly to the

mass-energy content of the universe while being

invisible (Luo, 2024). Understanding its nature could

unlock answers to some of the most sophisticated

questions in cosmology and particle physics,

potentially revealing physics beyond the Standard

Model (Queiroz, 2017).

This essay will explore three major theoretical

candidates for dark matter, i.e., Weakly Interacting

Massive Particles (WIMPs), axions, and primordial

black holes, examining their theoretical foundations,

physical characteristics, and current detection

strategies (Alonso-Álvarez & Tait, 2023). A

comparative analysis will then summarize the

strengths and limitations of each model using

contemporary data. Finally, the essay will conclude

by highlighting the ongoing relevance of dark matter

research and its broader implications for cosmology

and particle physics (CERN, 2022).

2 DEFINITIONS OF DARK

MATTER

Dark matter is an invisible and hypothetical form of

matter that neither emits, absorbs, nor reflects

electromagnetic radiation, which makes it

fundamentally different from ordinary (baryonic)

matter (Dutta, 2018; CERN, 2022). Its elusive nature

renders it undetectable through conventional

electromagnetic observational methods, such as

telescopes or radiation detectors (Primack & Gross,

2000). Nevertheless, dark matter reveals its presence

indirectly through gravitational interactions,

influencing the motion of galaxies, gravitational

lensing, and the large-scale structure of the universe

(CERN, 2022; Workman et al., 2024).

General relativity alone cannot account for the

observed gravitational effects in galactic rotations

and cosmic expansion unless an added source of

unseen mass is assumed (Gordon, 2020). This

discrepancy has led to the conclusion that a

substantial amount of matter, dark matter, must exist

beyond what can be seen directly. As such, dark

matter plays a vital role in cosmological models and

the formation of large-scale cosmic structures

(Primack & Gross, 2000).

Dark matter is typically categorized as "cold,"

"warm," or "hot," depending on its free-streaming

length, which refers to the typical distance a particle

travels without scattering (Workman et al., 2024).

Hot dark matter consists of fast-moving, low-mass

particles, such as relic neutrinos. Warm dark matter

particles, such as sterile neutrinos, are intermediate in

mass and speed, while cold dark matter, including

Weakly Interacting Massive Particles (WIMPs), is

composed of heavy, slow-moving particles that are

highly favoured in theoretical models for structure

formation (Feng & Zhang, 2024; Kumar, 2024).

Despite the name, dark matter is not necessarily “dark”

but is better described as “transparent,” as it does not

interact with light at all (Dutta, 2018).

Among the leading candidates for dark matter are

WIMPs, axions, and even primordial black holes (Lu,

2022). WIMPs are particularly attractive due to their

predicted stability over cosmic timescales and their

compatibility with cold dark matter models (Dey,

2023). These particles are thought to interact only via

gravity and the weak nuclear force, making them

detectable in principle through nuclear recoil events

in underground detectors or indirectly through their

annihilation products (Fox, 2018; Luo, 2024).

Despite ongoing experimental efforts, the true

appearance of dark matter remains one of the most

significant open questions in modern astrophysics

and particle physics.

3 WEAKLY INTERACTING

MASSIVE PARTICLES

Weakly Interacting Massive Particles (WIMPs) are

currently the most widely recognized candidates for

dark matter, and it is predicted that it makes up most

of the dark matter (Queiroz, 2017; O’Hare, 2017).

These hypothetical subatomic particles are massive

and electromagnetically neutral, meaning they do not

emit, absorb, or reflect light, making them nearly

impossible to detect using traditional methods (Dey,

2023). WIMPs are believed to be non-baryonic since

they are not composed of quarks and interact

primarily through gravity and potentially via other

weak-scale forces (Gordon, 2020; Kumar, 2024).

Searching for Dark Matter: Evidence from WIMPs, Axions, and Primordial Black Holes

253

In the early universe, when particles were in a

state of thermal equilibrium and dark matter and its

components existed in extra-dimensions, the majority

of WIMP candidates are predicted to have been

thermally created (Feng & Zhang, 2024; Queiroz,

2017). As the universe expanded and cooled, these

particles fell out of equilibrium and their abundance

"froze out", a process known as thermal freeze-out

(Feng & Zhang, 2024). WIMPs would have stopped

annihilating, and their abundance became fixed as the

universe cooled (Feng & Zhang, 2024). For a particle

with weak-scale mass and interaction strength, the

calculated abundance from this freeze-out matches

the dark matter density one observes today. Notably,

WIMPs arise naturally in many supersymmetric

extensions of the Standard Model, requiring no fine-

tuning to match cosmological data (Lu, 2022; Strumia,

2023).

Efforts to detect WIMPs fall into three primary

categories: direct detection, indirect detection, and

collider production. Direct detection involves

observing WIMPs interacting with atomic nuclei in

laboratory settings, while indirect detection focuses

on finding the secondary particles—such as gamma

rays, cosmic rays, and neutrinos—resulting from

WIMP annihilation or decay far from Earth (Gordon,

2020; Lu, 2022). Searches for WIMP annihilation,

such as gamma rays, neutrinos, and cosmic rays in

neighbouring galaxies and galaxy clusters, are part of

experimental efforts to find WIMPs (Gordon, 2020;

Workman et al., 2024). The Large Hadron Collider

(LHC) is also being used in attempts to produce

WIMPs directly through high-energy particle

collisions (Workman et al., 2024).

Indirect detection methods focus on regions

where dark matter is expected to accumulate, such as

galactic centres and dwarf galaxies. These areas have

low baryonic matter content, which minimizes

interference from ordinary astrophysical processes

(Lu, 2022). Telescopes like Fermi-LAT and

VERITAS have placed constraints on WIMP models

by failing to detect gamma rays that would result from

annihilation events. While high-energy neutrinos are

also predicted as annihilation products, their weak

interaction makes detection extremely difficult. The

ICECube neutrino observatory in Antarctica may

eventually distinguish WIMP-produced neutrinos

from background sources, though as of 2014, only 37

cosmic neutrinos had been recorded, making such

analysis inconclusive. Another possible indirect

signal may originate from the Sun. WIMPs

interacting with solar protons or helium nuclei may

lose energy and become gravitationally trapped if it

drops below the local escape velocity, potentially

contributing to a unique signal detectable by future

solar observations (Gordon, 2020).

4 AXIONS

Axions were once thought to provide a dynamical

solution to the Standard Model's strong CP problem

(Preskill, 2023). These particles contribute to the

cosmological constants, making them a practical

candidate for dark matter (Rosenberg, 2024; Svrcek

& Witten, 2023). Axion masses are typically

extremely small, often well below 1 eV, which means

they are unlikely to be produced in accelerators

(Rosenberg, 2024). Nevertheless, researchers

continue to experiment with innovative methods to

detect axions. These methods include:

 Laboratory-based detection: This involves

searching for axions or axion-induced effects

within laboratory settings. Utilising advanced

experimental setups, such as high-field magnets,

superconductors, low-background detection

systems, and low-radioactivity procedures is

necessary to detect them. Direct detection

methods are employed, where axions are

produced naturally in the laboratory. One such

method includes using high-density photon

sources like lasers to generate axions in a

magnetic field (Svrcek & Witten, 2023).

Another technique involves examining changes

in the polarization of laser beams passing

through a magnetic field (Rosenberg, 2024).

 Axion halo experiments: These experiments

aim to detect the axions thought to constitute the

local galactic dark matter halo. If axions are the

primary component of this halo, their number

density would be significant (Svrcek & Witten,

2023). The virtual velocity of these particles

within the galaxy is approximately 300 km/s, a

value that is consistent with the properties of

axions (Rosenberg, 2024). Axions are believed

to gain their velocity by falling into the galactic

potential well. If axions exist, they would be

produced in substantial quantities within the

solar interior (Svrcek & Witten, 2023).

 Axion helioscopes: These experiments search

for axions emitted by the Sun, which are

detected at terrestrial detectors. This involves

the conversion of solar axions back into photons

in a strong laboratory magnet. When the magnet

is oriented toward the Sun, X-rays can be

detected on the opposite side, as the resulting

photons retain the same energy as the incoming

axion. This phenomenon occurs through the

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Primakoff axion-photon conversion, where

axions transform into photons within the

Coulomb fields of charged particles in the solar

plasma (Rosenberg, 2024).

Axions can carry energy out of stars and are

created in hot astrophysical plasmas. The coupling

strength of these particles with normal matter and

radiation is bounded by the constraint that stellar

lifetimes or energy loss rates are not in conflict with

observational data (Preskill, 2023). There are two

main types of cosmic relic axion populations: a non-

thermal population, which behaves as cold dark

matter (CDM), and a thermal population, which

behaves as a hot dark matter (HDM) component,

similar to massive neutrinos. If axions were in

thermal equilibrium during the early Universe, this

would depend on the inflationary reheat temperature

and assumptions about pre-BBN cosmology. The

excess radiation generated in this process imposes

additional constraints on the axion-photon coupling

and on the mass of axions (Rosenberg, 2024; Svrcek

& Witten, 2023). Fig 1 below gives an illustration of

the Primarkoff axion-photon coupling (Irastorza,

2021).

Figure 1: Showing the Primarkoff axion-photon coupling

(Irastorza, 2021).

5 PRIMORDIAL BLACK HOLES

A type of hypothetical black holes, primordial black

holes (PBHs), are believed to have formed in the early

universe, shortly after the Big Bang, and they are a

fascinating candidate for dark matter. Unlike stellar

black holes that form from the gravitational collapse

of massive stars, PBHs are theorized to originate from

the direct collapse of high-density regions in the

infant universe before any stars even existed (Gordon,

2020). This makes them unique among black hole

classes and particularly interesting from a

cosmological standpoint. PBHs may have formed

during the radiation-dominated era, when the

universe was filled with high-energy particles and

extreme densities. During this time, quantum

fluctuations produced during the inflationary epoch

could have grown sufficiently large to cause localised

regions of spacetime to undergo gravitational

collapse once they re-entered the cosmological

horizon (Primark & Gross, 2000; Fox, 2018). These

regions, if spherically symmetric and dense enough,

could bypass the need for stellar evolution and

directly collapse into black holes of varying masses.

The masses of PBHs could span a wide range, from

sub-planetary scales to several hundred or even

thousands of solar masses, depending on the horizon

scale at the time of formation (Workman et al., 2024).

The collapse of super-horizon fluctuations,

extreme perturbations generated during inflation, is

believed to be the most probable origin of PBHs.

These fluctuations could act as seeds for black hole

formation in the early universe. PBHs could have

formed during both the inflationary and radiation-

dominated eras, particularly in regions where

subatomic matter was so densely packed that

gravitational collapse could, without the explosive

stellar supernova events associated with ordinary

black holes (Workman et al., 2024; Primack & Gross,

2000; Fox, 2018).

One feature that makes PBHs fit the primary

characteristic of dark matter is that they do not emit

light and are extremely difficult to detect via

electromagnetic radiation, and most importantly they

are non-luminous and interact predominantly through

gravity. Due to its close alignment with what is

expected from dark matter, PBHs are an attractive

dark matter candidate (Particle Data Group, 2012).

PBHs are supported historically by the MACHO

(Massive Astrophysical Compact Halo Object)

project in the 1990s. During the experiment, more

observed gravitational microlensing events were

happening towards the Magellanic Clouds than could

be explained by known stellar populations. This

implies that there is a presence of compact, invisible

objects in the galactic halo which have consistent

mass range with PBHs.

Recent advances in gravitational wave detection

have further revived interest in PBHs. NASA’s LIGO

(Laser Interferometer Space Antenna) have detected

many black hole mergers, some involving blackholes

with masses and properties that do not align with

standard stellar evolution models. These observations

have prompted the hypothesis that at least some of

these mergers could involve PBHs formed in the early

universe (Workman et al., 2024; Dey, 2023). In

Searching for Dark Matter: Evidence from WIMPs, Axions, and Primordial Black Holes

255

addition, LISA is expected to detect the upcoming

space-based gravitational wave detector and is

predicted to provide crucial data on mergers

involving lower mass blackholes. PBHs are a viable

dark matter candidates for several reasons:

 No need for new particles: Unlike WIMPS or

axions, PBHs do not require the introduction of

any new fundamental particles beyond the

standard model. Their formation is a natural

phenomenon of general relativity applied to

conditions in the early Universe.

 Gravitational dominant: PBHs interact via

gravity, which is how dark matter influences

large-scale structures like galaxies and galaxy

clusters.

 Longevity: PBHs with masses above 10

grams

would not have evaporated due to Hawking

radiation, making them stable on cosmological

time scales (Workman et al., 2024).

However, PBHs are not without their challenges.

So far, observational methods like microlensing

surveys (e.g., EROS, OGLE), cosmic microwave

background (CMB) distortion studies, and wide

binary start constraints have not yet had much

evidence of primordial black holes. Nevertheless,

there remain several “mass windows” in which could

still make up all or sizeable proportion of dark matter.

For example, asteroid-mass PBHs (~10

to 10

grams) and intermediate-mass PBHs (~10-100 solar

masses) remain less constrained as one does not yet

have strong enough data in those ranges to refuse the

existence of PBHs (Workman et al., 2024; Carr, n.d.).

Current and future observational programs such

as the Vera C. Rubin Observatory’s Legacy Survey of

Space and Time (LSST), the Square Kilometer Array

(SKA), and space-based gravitational wave

observatories like LISA, are expected to test these

remaining possibilities more deeply. Improved

stimulations of early-universe density fluctuations

and more detailed modelling of PBH formation

scenarios are also helping theorists narrow down

workable models (Harigaya & Lou, 2023; Strumia,

2023). Some researchers have even proposed broader

cosmological roles for PBHs. PBHs might explain the

origin of supermassive black holes in galaxy centers,

contribute to the reionisation of the universe, or even

play a part in generating matter-antimatter asymmetry

through Hawking radiation (Alonso-Álvarez & Tait,

2023).

In conclusion, while PBHs have not yet been

detected, they are still a strong candidate for

explaining dark matter due to their compatibility with

existing cosmological theories, combined with their

potential detectability through gravitational wave

astronomy, ensuring that they will remain a focus of

intense research in the years to come.

6 COMPARISON, LIMITATIONS

AND PROSPECTS

Despite the abundant gravitational evidence in

galaxies, the true nature of dark matter is still

unknown (CERN, 2022). WIMPs, Axions, and

Primordial black holes are currently the top

competitors; each has its own advantages and

emerges from different theoretical motivations. They

interact with gravity and the weak nuclear force, and

they are heavy. They are created in the early cosmos

through thermal production, in which particles

separate from the heated plasma of the early universe

(Lu, 2022). Nevertheless, despite decades of

searching via indirect detection (gamma rays,

neutrinos), collider experiments (LHC), and

underground detectors (LUX, XENON), WIMPs

have not yet been found. In the future, more sensitive

detectors and background investigations of

gravitational waves will be used. In contrast, axions

have a mass density spectrum that is 10

-15

times that

of WIMPs. They were first put forth to address the

strong CP problem in quantum chromodynamics, but

because of their similarities to the expected dark

matter candidates, they gained popularity as dark

matter candidates (Workman et al., 2024). However,

so far there is not any evidence of WIMPs due to their

substantially light mass and weak coupling

interactions with photons and matter makes it

significantly hard to detect in space. Active search

ongoing and new techniques like halo scopes and

quantum sensor are emerging. Finally, Primordial

Blackholes (PBHs), although not the most widely

accepted model, does not require new particles. They

are formed from density fluctuations in the early

universe with a mass of several solar masses and only

interact gravitationally and it has the greatest number

of detecting methods such as gravitational lensing,

CMB effects, gravitational waves, dynamical tests. It

is challenging to differentiate it from astrophysical

black holes (Workman et al., 2024). Current

blackhole detection techniques will continue to find

primordial blackholes. WIMPs is the front-runner,

even though each candidate addresses some

important physics issues.

7 CONCLUSIONS

To sum up, this study examined three prominent dark

matter candidates, i.e., WIMPs, axions, and PBHs,

IAMPA 2025 - The International Conference on Innovations in Applied Mathematics, Physics, and Astronomy

256

comparing their origins, physical properties,

detection strategies, and current experimental

constraints. WIMPs, while theoretically attractive

due to the “WIMP miracle”, faces challenges from

decades of blank results. Axions, although offers an

elegant pair of solution to the strong CP problem and

dark matter, their extraordinarily weak interactions

make them almost impossible to detect. PBHs, offers

a non-particle alternative tied to early-universe

conditions, yet are increasingly constrained by

observational data. Each candidate occupies a unique

theory, highlighting the need for diverse approaches.

This comparative study underscores the importance

of pursing multiple approaches in the dark matter

search. By mapping the strength and limitations of

leading candidates, this work contributes to

reminding future strategies in both theoretical

modelling and experimental design. Looking forward,

advances in detector sensitivity, gravitational wave

astronomy, and cosmological observations will

continue to experiment and refine the viable

parameter space for each candidate. The harmony

between theory, experiment, and astrophysical data

will be key in guiding discovery.

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