2-3
Theoretical classifications
2-3
a
Composition
There
are various hypotheses about what dark matter could consist of,
as
set out in the table below.
Dark matter can refer to any substance which interacts predominantly
via
gravity with visible matter (e.g., stars and planets). Hence in
principle it
need not be composed of a new type of fundamental particle but
could, at
least in part, be made up of standard baryonic matter, such as
protons or
neutrons. However, for the reasons outlined below, most scientists
think
the dark matter is dominated by a non-baryonic component, which
is likely
composed of a currently unknown fundamental particle (or similar
exotic state).
2-3
b
Baryonic matter
Not to be confused with Missing baryon problem.
Baryons (protons and neutrons) make up ordinary stars and planets.
Ho47
wever, baryonic matter also encompasses less common non-primordial
black holes, neutron stars, faint old white dwarfs and brown dwarfs,
collectively
known as massive compact halo objects (MACHOs), which can
be hard to detect.
However, multiple lines of evidence suggest the majority of dark
matter
is not made of baryons:
Sufficient diffuse, baryonic gas or dust would be visible when
backlit by
stars.
The theory of Big Bang nucleosynthesis predicts the observed abundance
of the chemical elements. If there are more baryons, then there
should also
be more helium, lithium and heavier elements synthesized during
the Big
Bang. Agreement with observed abundances requires that baryonic
matter
makes up between 4–5% of the universe’s critical density.
In contrast,
large-scale structure and other observations indicate that the
total matter
density is about 30% of the critical density.
Astronomical searches for gravitational microlensing in the Milky
Way
found at most only a small fraction of the dark matter may be
in dark,
compact, conventional objects (MACHOs, etc.); the excluded range
of
object masses is from half the Earth’s mass up to 30 solar
masses, which
covers nearly all the plausible candidates.
Detailed analysis of the small irregularities (anisotropies) in
the cosmic
microwave background. Observations by WMAP and Planck indicate
that
around five-sixths of the total matter is in a form that interacts
significantly
with ordinary matter or photons only through gravitational effects.
