There are many reasons to believe that the universe is full of
matter that influences the evolution of the universe gravitationally, but is
not seen directly
in our present observations.
FIGURE: Superposed on
an optical picture of a group of galaxies is an
X-ray image taken by
The image shows hot gas (which produces X-rays)
false red color (Ref).
The presence of this confined gas
indicates that the gravity
in groups and clusters of galaxies is larger than that expected from the
matter that we can observe in those galaxies.
The adjacent image exhibits one recent piece of
evidence for undetected
matter: the hot gas seen in the X-ray spectrum would have
dispersed if it were held in place only the by gravity of the mass that is
producing light in this image (the so-called "luminous mass").
The nature of this dark matter, and the
associated "missing mass problem", is one of the
fundamental cosmological issues of
Hot Dark Matter and Cold Dark Matter
Discussions of dark matter typically consider two extremes
Hot dark matter is composed of particles that have zero or near-zero mass (the
neutrinos are a prime example). The Special Theory of Relativity requires that
massless particles move at the speed of light and that
nearly massless particles move at
nearly the speed of light. Thus, such very low mass particles must move at very
high velocities and thus form (by the kinetic theory of gases) very hot gases.
- Hot Dark Matter
Cold Dark Matter
On the other hand, cold dark matter is composed of objects sufficiently
massive that they move at sub-relativistic velocities. They thus form much colder
gases. The difference between cold dark matter and hot dark matter is significant
in the formation of structure, because the high velocities of hot dark matter cause
it to wipe out structure on small scales.
Tutorial on Current Status of Dark Matter
The following is a brief tutorial on this
- If inflation
is correct the density of the Universe should be exactly the closure density.
Luminous stars and galaxies
contribute only about 0.5% of the closure density, so 99% of the
Universe is in the form of dark matter.
We may speculate on what particles could make up this dark matter.
The known neutrinoes
have problems as candidates for dark matter because they
are relativistic (hot dark matter) and therefore they erase
fluctuations on small scales.
Thus, relativistic neutrinos could form large structures like superclusters, but
would have trouble forming smaller structures like galaxies. These arguments might
be at least partially invalidated if one of the types of neutrinos (the tau
neutrino is the obvious candidate) is considerably more massive than the electron
or muon neutrino.
- On smaller scales such as galaxies and clusters of galaxies, dynamical
estimates of the mass based on rotation curves or
velocity dispersions of galaxies indicate that 90% (not 99%)
of the total mass is not seen ("sub-luminous").
This implies that the mass density of the Universe is
10% of the closure density. In this case, the sub-luminous mass could
be normal (baryonic) and be locked up in stellar remnants
(white dwarfs, neutron stars, black holes) or just in very dim stars called
There is recent evidence for possible observation of one of these very dim
- Although inflation demands that the Universe have a density equal
to its critical density (and inflation is necessary to solve certain problems of
the standard big bang model like the horizon
problem) there has never been any observational evidence to support
this high of mass density. Most dynamical studies suggest values of
10-20% of closure density. These studies are based on large scale
deviations from Hubble expansion velocities (peculiar
- Large scale structure (e.g. the distribution of galaxies) is very
hard to understand, particularly in light of the relatively smooth
microwave background as measured by the COBE satellite.
One way to accomodate this is
to go to a mixed dark matter model in which you have some hot dark matter
(for the large scale) and some cold dark matter
to act as a seed for galaxy
formation. None of those models, however, fit the data using the
critical density. The best models to date
suggest mixed dark matter and an
overall cosmological mass density of 20-30% of closure. Hence, to
retain inflation, with its inescapable prediction that the Universe
must be flat, requires re-invoking Einstein's cosmological constant -
meaning the universe has vacuum energy (negative pressure) and is
currently accelerating. This makes our cosmology complicated but much
data is pointing this way.
- Supernova 1987a neutrino time of flight studies as well as the Solar
Neutrino experiment are consistent with the neutrino having a mass, but
a very small mass, not one that can cosmologically dominate.
We cannot currently test for various supersymmetric particles which
would only be created at very high energy (e.g. the early universe) -
so there remain many viable potential particles that are consistent with
the Standard Model of particle physics,
that would remain unnoticed in any accelerator experiments.
Searches for Dark Matter Candidates
Here are links to two experimental searches for dark matter candidates that could
be made of ordinary matter (what astromomers call baryonic matter):
For a more extensive discussion of dark matter, see
These particular searches make use of the principle of
gravitational lensing in the
theory of General Relativity.
Finally, do not confuse the term "dark matter" with the term "antimatter". Here is a
discussion of the difference.