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Non-baryonic matter

Candidates for non-baryonic dark matter are hypothetical particles such as
axions, sterile neutrinos, weakly interacting massive particles (WIMPs),
gravitationally-interacting massive particles (GIMPs), supersymmetric
particles, geons, or primordial black holes. The three neutrino types already
observed are indeed abundant, and dark, and matter, but because their
individual masses – however uncertain they may be – are almost certainly
too tiny, they can only supply a small fraction of dark matter, due to limits
derived from large-scale structure and high-redshift galaxies.
Unlike baryonic matter, nonbaryonic matter did not contribute to the formation
of the elements in the early universe (Big Bang nucleosynthesis)
and so its presence is revealed only via its gravitational effects, or weak
lensing. In addition, if the particles of which it is composed are supersymmetric,
they can undergo annihilation interactions with themselves, possibly
resulting in observable by-products such as gamma rays and neutrinos
(indirect detection).
Dark matter aggregation and dense dark matter objects
If dark matter is composed of weakly-interacting particles, an obvious
question is whether it can form objects equivalent to planets, stars, or black
holes. Historically, the answer has been it cannot, because of two factors:
It lacks an efficient means to lose energy
Ordinary matter forms dense objects because it has numerous ways to lose
energy. Losing energy would be essential for object formation, because
a particle that gains energy during compaction or falling “inward” under
gravity, and cannot lose it any other way, will heat up and increase velocity
and momentum. Dark matter appears to lack means to lose energy, simply
because it is not capable of interacting strongly in other ways except
through gravity. The virial theorem suggests that such a particle would not
stay bound to the gradually forming object – as the object began to form
and compact, the dark matter particles within it would speed up and tend
to escape.
It lacks a range of interactions needed to form structures
Ordinary matter interacts in many different ways. This allows the matter
to form more complex structures. For example, stars form through gravity,
but the particles within them interact and can emit energy in the form of
neutrinos and electromagnetic radiation through fusion when they become
energetic enough. Protons and neutrons can bind via the strong interaction
and then form atoms with electrons largely through electromagnetic
interaction. But there is no evidence that dark matter is capable of such a
wide variety of interactions, since it seems to only interact through gravity
(and possibly through some means no stronger than the weak interaction,
although until dark matter is better understood, this is only hopeful speculation).
In 2015–2017 the idea dense dark matter was composed of primordial black
holes, made a comeback following results of gravitational wave measurements
which detected the merger of intermediate mass black holes. Black
holes with about 30 solar masses are not predicted to form by either stellar
collapse (typically less than 15 solar masses) or by the merger of black holes
in galactic centers (millions or billions of solar masses). It was proposed
the intermediate mass black holes causing the detected merger formed in
the hot dense early phase of the universe due to denser regions collapsing.
A later survey of about a thousand supernovae detected no gravitational
lensing events, when about eight would be expected if intermediate mass
primordial black holes above a certain mass range accounted for the majority
of dark matter.
The possibility atom-sized primordial black holes account for a significant
fraction of dark matter was ruled out by measurements of positron and
electron fluxes outside the Sun’s heliosphere by the Voyager 1 spacecraft.
Tiny black holes are theorized to emit Hawking radiation. However, the
detected fluxes were too low and did not have the expected energy spectrum
suggesting tiny primordial black holes are not widespread enough
to account for dark matter. Nonetheless, research and theories proposing
dense dark matter accounts for dark matter continue as of 2018, including
approaches to dark matter cooling, and the question remains unsettled. In
2019, the lack of microlensing effects in the observation of Andromeda
suggests tiny black holes do not exist.
However, there still exists a largely unconstrained mass range smaller than
that can be limited by optical microlensing observations, where primordial
black holes may account for all dark matter.