The existence of ``dark matter'' is inferred from astrophysical
observations that probe gravitational potentials. The mass
content required to provide the derived gravitational potential
is then compared with the visible mass content. Several types of
observation allow this to be done and in most cases the mismatch
between the required mass and the observed mass is extreme. The
following list summarises some of the evidence that has been
accumulated:
Studies of the dynamics of stars in the local disk
environment gave rise to the first suggestion of `missing
matter' nearly 70 years ago [99,
100]. The kinetic energy associated with the motion of these stars
normal to the plane of the Milky Way gives a measure of the
restraining gravitational potential that binds them to the
disk. Since the first work by Oort, a number of further studies
have given conflicting results. However, even if present, this
particular disk dark matter is not significant compared with
the halo component.
Rotation curves for a large number of spiral galaxies have
now been reliably established and it is observed that the
orbital velocities of objects (stars, globular clusters, gas
clouds,
etc.) tend to a constant value, independent of the radial position
r, even for objects out toward, and even far beyond, the edge of
the visible disks. This is quite inconsistent with the
behaviour expected from Newtonian mechanics, assuming most
mass is in the central part of the galaxies. According to
Newtonian mechanics, the mass density within these galaxies is
only declining as
, leading to a total mass that actually continues to increase
proportional to
r
.
Within the Local Group of galaxies, the Milky Way and
Andromeda (M31) are approaching each other at a much faster
pace than can be explained by gravitational attraction of the
visible mass. To explain the approach velocity, and indeed the
fact that these two galaxies are not still moving away from
each other as part of the Hubble expansion, requires each to
have masses that are consistent with those deduced from their
rotation curves.
Many clusters of galaxies show extended x-ray emission.
This is usually attributed to a thin plasma of hot gas. On the
assumption that the hot gas is gravitationally bound to the
cluster and in equilibrium (i.e.
we have a virial system), the gravitational potential energy
can be inferred from the kinetic energy budget of the hot gas.
The cluster mass determined in this way is much higher than
that seen either visibly or in the gas itself.
Gravitational lensing by clusters of galaxies causes images
of more distant galaxies to be distorted and often split into
multiple images. The gravitational mass of the lens (i.e.
the cluster), and its distribution, can be recovered through
detailed analysis of the image pattern surrounding the cluster.
The lenses show a far more extended spatial extent than the
visible cluster.
Galaxy red-shift surveys have revealed large-scale
galaxy-cluster streaming motions superimposed on the Hubble
expansion. Attempts to explain this due to gravitational
attraction resulting from the overall distribution of galaxy
superclusters give the right direction of motion but need more
than the observed visible masses in the superclusters to
explain the speed of motion.
The next four items are not really at the same level of
``simple'' observational evidence as those above, as they require
reliance on a more convoluted path to determine masses involved.
However, the first three of these have received a great deal of
effort and are now heavily used as a combined strong argument in
favour of the existence of ``dark matter'', and indeed have
resulted in a consensus view of ``standard cosmology'' over the
past few years.
The large scale structure (LSS) of the Universe can be
studied using large surveys of distant galaxies, by measuring
their spatial distribution and peculiar motions. There is an
extensive industry in
N
-body simulations trying to explain the LSS and large-scale
dynamics in terms of gravitational growth of small
perturbations present in the early Universe. The only
simulations that give reasonable agreement with observation are
those that use a matter density somewhat higher than currently
thought allowable in visible matter. Indeed, starting from the
level of the COBE observations of the density fluctuations () at the time of recombination (z=1000), for gravitational
instability to lead to galaxy formation on a reasonable
timescale it seems necessary to invoke a significant dark
matter component, which only interacts gravitationally.
Type Ia supernovae can be used as standard candles to
determine distances, independently of red-shift to high
red-shift galaxies in which they occur. This allows the
geometry of space-time to be studied at high red-shift. The
implications of the results will be discussed later, but
consistent cosmological models seem to require a dark matter
component.
The COBE satellite gave us the first measurement of the
amplitude of microwave background anisotropies at the time of
recombination. It is these perturbations which subsequently
grow through gravitational instabilities to form the
large-scale structure seen today. COBE had a relatively poor
angular resolution. Recently, new results have determined the
angular power spectrum of the microwave background anisotropies
at much finer angular scales, where enhancements are expected
due to acoustic wave resonances in the early Universe. The
position and amplitude of the enhancement depends on parameters
of the early Universe. A clear first peak is seen in the data
and its position favours a dark matter component. Even second
and third peaks look to be emerging and the amplitudes and
positions of these provide constraints on various cosmological
parameters (this will be discussed in more detail in the
following section).
For those who believe in inflation, most surviving models
naturally have a density equal to the critical density, which
exceeds that possible in visible matter.
With such a large volume of evidence there can be no doubt that
there is a real mystery to be unravelled here. Ideally, it would
be satisfying if there were a single simple solution that
explained all the above. This has proven elusive so far, but
recently there has been some convergence on models that address
the larger scale issues to do with the Universe as a whole, and
this is discussed in the next subsection. The main aim is to
establish a consensus opinion on the dark matter fraction, and
more specifically the cold dark matter fraction, as this
motivates most of the experimental searches for dark matter. In
doing this we will see that a strong argument for a standard
cosmology, with cold dark matter as one of its components, is
beginning to become established. However, some issues clearly
hint at aspects of the cosmology that have yet to be properly
resolved, and some of these do have potentially serious
implications for the cold dark matter component. These will be
discussed in section
2.3
.