The dark side of cosmology: Dark
matter and dark energy
David N. Spergel*
A simple model with only six parameters (the age of the universe, the density of atoms,
the density of matter, the amplitude of the initial fluctuations, the scale dependence of this
amplitude, and the epoch of first star formation) fits all of our cosmological data . Although
simple, this standard model is strange. The model implies that most of the matter
in our Galaxy is in the form of “dark matter,” a new type of particle not yet detected
in the laboratory, and most of the energy in the universe is in the form of “dark energy,”
energy associated with empty space. Both dark matter and dark energy require extensions
to our current understanding of particle physics or point toward a breakdown of general
relativity on cosmological scales.
John Archibald Wheeler, my academic great- grandfather, succintly summarized “geom- etrodynamics,” his preferred name for the theory of general relativity (1): “Spacetime tells matter how to move; matter tells spacetime how to curve.”
Cosmologists observe the motion of atoms (
either in the form of gas or stars) or follow the
paths taken by light propagating across the universe and use these observations to infer the
curvature of spacetime. They then use these measurements of the curvature of spacetime to infer
the distribution of matter and energy in the universe. Throughout this Review I will discuss a variety of observational techniques, but ultimately they
all use general relativity to interpret the observations and they all lead to the conclusion that
atoms, stuff that we understand, make up only 5%
of the matter and energy density of the universe.
Standard cosmological model fits,
but at a price
Observations of the large-scale distribution of
galaxies and quasars show that the universe is
nearly uniform on its largest scales (2) and that
the velocity of a distant galaxy depends on its
distance (3). General relativity then implies that
we live in an expanding universe that started in a
big bang. Because the universe expands, light is
“redshifted,” so that light from a distant galaxy
appears redder when it reaches us. Hubble’s observations that found a linear relationship between
galaxy redshift and distance established the basic
model in the 1920s.
Our current cosmological standard model as-
sumes that general relativity and the standard
model of particle physics have been a good de-
scription of the basic physics of the universe
throughout its history. It assumes that the large-
scale geometry of the universe is flat: The total
energy of the universe is zero. This implies that
Euclidean geometry, the mathematics taught to
most of us in middle school, is valid on the scale
of the universe. Although the geometry of the
universe is simple, its composition is strange:
The universe is composed not just of atoms (mostly
hydrogen and helium), but also dark matter and
The currently most popular cosmological model posits that soon after the big bang, the universe underwent a period of very rapid expansion.
During this inflationary epoch, our visible universe
expanded in volume by at least 180 e-foldings.
The cosmic background radiation is the leftover
heat from this rapid expansion. This inflationary
expansion also amplifies tiny quantum fluctuations into variations in density. The inflationary
model predicts that these fluctuations are “
nearly scale-invariant”: The fluctuations have nearly
the same amplitude on all scales.
These density variations set off sound waves
that propagate through the universe and leave
an imprint in the microwave sky and the large-scale distribution of galaxies. Our observations of
the microwave background are a window into the
universe 380,000 years after the big bang. During
this epoch, electron and protons combined to
form hydrogen. Once the universe became neutral,
microwave background photons could propagate
freely, so the sound waves imprint a characteristic scale, the distance that they can propagate in
380,000 years. This characteristic scale, the “baryon
acoustic scale,” serves as a cosmic ruler for measuring the geometry of space, thus determining
the density of the universe.
Observations of the temperature and polarization
fluctuations in the cosmic microwave background,
both from space (4–6) and from ground-based
telescopes (7, 8), test this standard cosmological
model and determine its basic parameters. Remarkably, a model with only six independent
parameters—the age of the universe, the density
of atoms, the density of matter, the amplitude of
the density fluctuations, their scale dependence,
and the epoch of first star formation—provides a
detailed fit to all of the statistical properties of
the current microwave background measurements.
The same model also fits observations of the
large-scale distribution of galaxies (9), measure-
ments of the Hubble constant, and the expansion
rate of the universe (10, 11), as well as distance
determinations from supernovae (12). The suc-
cess comes at a price: Atoms make up less than
5% of our universe; the standard model posits
that dark matter dominates the mass of galaxies
and that dark energy, energy associated with
empty space, makes up most of the energy den-
sity of the universe (see Fig. 1).
Astronomical observations and cosmological
theory suggest that the composition of the universe is remarkably rich and complex. As Fig.
1 shows, the current best estimates of the universe’s composition (5–8) suggest that dark energy, dark matter, atoms, three different types of
neutrinos, and photons all make an observable
contribution to the energy density of the universe.
Although black holes are an unlikely candidate
for the dark matter (13), their contribution to the
mass density of the universe is roughly 0.5% of
the stellar density (14).
Astronomical evidence for dark matter
The evidence for dark matter long predates our
observations of the microwave background, supernova observations, and measurements of large-scale structure. In a prescient article published in
1933, Fritz Zwicky (15) showed that the velocities
of galaxies in the Coma cluster were much higher
than expected from previous estimates of galaxy
masses, thus implying that there was a great deal
of additional mass in the cluster. In the 1950s,
Kahn and Woltjer (16) argued that the Local
Group of galaxies could be dynamically stable
only if it contained appreciable amounts of unseen matter. By the 1970s, astronomers argued
that mass in both clusters (17) and galaxies (18)
increased with radius and did not trace light.
Theoretical arguments that showed that disk
stability required dark matter halos (19) buttressed
these arguments. Astronomers studying the motion
of gas in the outer regions of galaxies found
evidence in an ever-increasing number of systems
for the existence of massive halos (20–24). By the
1980s, dark matter had become an accepted part
of the cosmological paradigm.
What do we know about dark matter from
astronomical observations today?
Microwave background and large-scale structure observations imply that dark matter is five
times more abundant than ordinary atoms (4–8).
The observations also imply that the dark matter
has very weak (or no) interactions with photons,
electrons, and protons. If the dark matter was
made of atoms today, then in the early universe,
it would have been made of ions and electrons
and would have left a clear imprint on the microwave sky. Thus, dark matter must be nonbaryonic and “dark.”
Observations of large-scale structure and simulations of galaxy formation imply that the dark
matter must also be “cold”: The dark matter particles must be able to cluster on small scales.
Simulations of structure formation with cold dark
matter (and dark energy) are generally successful
at reproducing the observations of the large-scale
distribution of galaxies (25). When combined
with hydrodynamical simulations that model
the effects of cooling and star formation, the
1100 6 MARCH 2015 • VOL 347 ISSUE 6226 sciencemag.org SCIENCE
Princeton University, Princeton, NJ 08544, USA.
*Corresponding author. E-mail: email@example.com