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If inflation
were correct and the cosmological constant were zero,
the matter density of the Universe would be exactly the closure density,
which would lead to flat geometry. Current data indicate that
the Universe is indeed flat, as predicted by inflation,
but that it does not contain a
closure density of matter because there is a non-zero cosmological
constant. Instead, about 30 percent of the closure
density is supplied by matter and about 70 percent by dark energy
(vaccum energy or a cosmological constant).
Luminous matter contributes a small fraction
of the closure density, implying that the vast majority of the
mass density is dark matter. Thus, the present Universe is dominated
by dark matter and dark energy.
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The known neutrinos
are relativistic (that is, they are hot dark matter) and therefore they erase
fluctuations on small scales.
They could aid the formation of large structures like superclusters but
not smaller structures like galaxies. Thus, they are not likely to account for
more than a small fraction of the dark matter. The
WMAP analysis
of cosmic microwave background fluctuations
indicates that light neutrinos contribute less than 2 percent of the
total energy density at decoupling.
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On the scale of galaxies and clusters of galaxies,
90 percent
of the total mass is not seen.
In this case, a significant fraction of the dark matter could
be normal (that is, baryonic) and be in the form of small,
very low luminosity objects like
white dwarfs, neutron stars, black holes, brown dwarfs, or red dwarfs.
However, microlensing
observations and searches for subluminous objects generally have not found enough of these
"normal" objects to account for the mass of galaxy halos.
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Data indicate a small mass for neutrinos, but not one large enough
to dominate the mass density of the Universe. Further, strong constraints
from big bang nucleosynthesis compared with the observed
abundances of the light elements indicate that most of the dark matter
is not baryonic. Thus, a significant fraction of the dark
matter is likely to be nonbaryonic and not neutrinos, and to be cold (that
is, massive so that it does not normally travel at relativistic velocities).
Current speculation centers on not yet discovered elementary particles as the
candidates for this cold dark matter.
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Large-scale structure and its rapid formation in the early Universe is
hard to understand, given the smallness of the cosmic microwave background
fluctuations implied by COBE and WMAP, unless cold dark matter plays
a central role in seeding initial structure formation.
The models of structure formation most consistent with current data are
probably the class of ΛCDM models that combine a cosmological
constant (denoted by Λ) with cold dark matter (CDM) to give
an accelerating but flat universe with cold dark matter to seed structure formation.
As a bonus, the finite
cosmological constant (with associated acceleration of the cosmic expansion)
that is implicit in these models
also makes the age of the Universe greater than we would
estimate otherwise, which may help erase
with any remaining discrepancies between the
age of the Universe and the age of its oldest stars.
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These observations taken together appear to justify several
general statements.
First, the Universe is flat and is presently dominated by dark
energy (finite cosmological constant) and dark matter. This
strongly favors the validity of the inflationary hypothesis.
Second, cold dark matter
probably was central to
the formation of structure. Third, most of the
dark matter is probably not "ordinary matter" (not baryonic).
Thus, the growing evidence is that we live in a Universe dominated
by dark energy and dark matter. We have as yet no strong clues
as to the source and detailed nature of either because neither
has been captured in a laboratory. At present, we know about dark matter
and dark energy only from observations on galactic and larger scales in
the cosmos.