simulations can reproduce the basic observed
properties of galaxies (26, 27).
Supermassive clusters are important laboratories for studying dark matter properties. These
clusters are thought to be “fair samples” of the
universe, as the ratio of dark matter to ordinary
matter observed in the clusters is very close to
the cosmological value (28). X-ray observations
directly trace the distribution of ordinary (“
baryonic”) matter as most of the atoms in the cluster
gas have been ionized. As Zwicky (29) first discussed, observations of gravitational lensing of
background galaxies directly trace the total distribution of matter in the clusters. Today, over 75
years after Zwicky’s suggestion, astronomers use
large-format cameras on the Hubble Space Telescope to make detailed maps of the cluster dark
matter distribution (30). These observations reveal considerable amounts of dark matter substructure in the clusters, generally consistent with
the predictions of numerical simulations (31).
At much smaller scales, dwarf galaxies are
another important astronomical testing ground
for theories of dark matter. The gravitational potential wells of these dark matter–dominated systems are quite shallow, so the predicted properties
of dwarf galaxy halos are quite sensitive to dark
matter properties. Several groups (32, 33) have
argued that the observed properties of dwarf
galaxies do not match the predictions of numerical simulations. Although some astrophysicists
argue that improved models of star-formation
feedback can reconcile this discrepancy (34), others
suggest that dark matter self-interactions are
needed to match simulations to observations (35).
All of the astronomical arguments for the existence of dark matter assume that general relativity is valid on galactic scales. Alternative gravity
theories, such as modified Newtonian dynamics
(MOND) (36), obviate the need for dark matter
by changing the physics of gravity. Although these
models have some phenomenological success on
the galaxy scale (37), they have great difficulties
fitting the microwave background fluctuation observations (4–8, 38) and observations of clusters,
particularly the bullet cluster (39). Most theorists
also consider these alternative models as lacking
motivation from fundamental physics.
What is the dark matter?
The existence of nonbaryonic dark matter implies that there must be new physics beyond the
standard model of particle physics. Particle physicists have suggested a wealth of possibilities,
some motivated by ideas in fundamental physics and others by a desire to explain astronomical phenomena (40).
The early universe was an incredibly powerful
particle accelerator. At the high temperatures
and densities of the early moments of the big
bang, the cosmic background radiation created
an enormous number of particles. Cosmic microwave background experiments (5–8) have detected
the observational signatures of the copious number of neutrinos produced in the early first moments of the universe. These early moments could
have also created the dark matter particles.
Supersymmetry, the most studied extension
of our current understanding of particle physics,
provides potential candidates for dark matter.
Particles can be divided into two types: fermions
and bosons. Fermions obey the Pauli exclusion
principle: Only one particle can be found in each
state. Multiple bosons can be found in the same
quantum state. Electrons are fermions, while photons are bosons. Supersymmetry would be a new
symmetry of nature that links each boson to a
fermionic partner and vice versa. This symmetry
implies a plethora of new particles: The photon
would have a fermionic partner, the photino, and
the electron would have a bosonic partner, the
selectron. One of the goals of the Large Hadron
Collider (LHC) is to search for these yet undiscovered supersymmetric particles.
The lightest supersymmetric particle (LSP) can
be stable. These particles would have been produced copiously in the first moments after the
big bang. For certain parameters in the supersymmetric model, the abundance of the LSP is just
what is needed to explain the observed abundance
of dark matter. This success is an example of the
“WIMP miracle” of cosmology: A weakly interacting massive particle (WIMP), a particle that
interacts through exchanging particle with masses
comparable to the Higgs mass, has the needed
properties to be the dark matter.
Particle physics suggests other well-motivated
dark matter candidates, including the axion
(41) and “asymmetric dark matter” (42), particles
whose abundances are not set by their cross
section but by an asymmetry between particles
If WIMPs are the dark matter, then they could
be detected through several different routes: Dark
matter could be created at an accelerator or
seen either in deep underground experiments
or through astronomical observations (40, 43).
These possibilities have led to an active program
of searching for dark matter. This search has had
many exciting moments. There are currently a
number of intriguing signals that might turn out
to be the first detection of dark matter:
1) The Gran Sasso Dark Matter (DAMA) experiment has seen an annual modulation in the
event rate in its detector (44) with just the theoretical predicted form (45). The interpretation
of this result is controversial, as other experiments have failed to detect dark matter and
seem to be in contradiction with this detection
claim (46, 47).
2) There have been multiple claims of excess
gamma-ray signals coming from the center of our
Galaxy at a range of potential dark matter masses
(48, 49). Because of the high dark matter density
in the galactic center, it is potentially the brightest
source of high-energy photons produced through
dark matter self-annihilation. However, the galactic center also contains a wealth of astrophysical
sources that emit high-energy photons. Searches
in external galaxies have also suggested the existence of dark matter with yet a different mass
(50). This claim is also controversial (51). Cosmologists hope that observations of nearby dwarfs
could provide a less ambiguous signal (52).
3) Dark matter annihilation in our Galaxy
could potentially produce positrons. Cosmic-ray
experiments have been searching for these signals (53). The challenge for these experiments is
to separate this signal from astrophysical sources
of cosmic rays, such as pulsars and production
from secondary collisions.
Hopefully, future experiments will verify one
of these results.
The multiple components that compose our universe
Current composition (as the fractions evolve with time)
Fig. 1. The multiple components that compose our universe. Dark energy comprises 69% of the
mass energy density of the universe, dark matter comprises 25%, and “ordinary” atomic matter makes up
5%. There are other observable subdominant components: Three different types of neutrinos comprise at
least 0.1%, the cosmic background radiation makes up 0.01%, and black holes comprise at least 0.005%.