9 Dark Matter
The analysis of anisotropies in the cosmic microwave background have shown that
the fraction훺c= 0 .263 of all the matter in the Universe is nonbaryonic. All forms
of radiating baryonic mass are already accounted for in훺b: starlight amounts to
훺∗= 0 .001–0.002, gas and star remnants in galaxies amount to훺lum< 0 .01. The inter-
galactic gas contains hydrogen clouds and filaments seen by its Ly훼absorption, warm
gas in groups of galaxies radiates soft X-rays, hot gas in clusters is seen in keV X-rays
and in the SZE. The missing fraction is not radiating in any wavelength and is there-
fore calleddark matter.
This term is generic for observed gravitational effects on all scales: galaxies, small
and large galaxy groups, clusters and superclusters, CMB anisotropies over the full
horizon, baryonic oscillations over large scales, and cosmic shear in the large-scale
matter distribution. The correct explanation or nature of dark matter is not known,
whether it implies unconventional particles or modifications to gravitational theory.
but gravitational effects have convincingly proved its existence in some form.
The remarkable fact is that the dark matter fraction is much larger than the known
fraction and that it does not interact with ordinary baryonic matter except gravita-
tionally. The purpose of this Chapter is to summarize the phenomenology of all such
effects except CMB which was covered in Chapter 8.
In Section 9.1 we study kinematical and dynamical effects: the virial theorem,
empirical halo profiles and examples of virially bound groups and clusters. In
Section 9.2 we study dark matter in galaxies: spirals, ellipticals, dwarf sphericals,
and some arguments on galaxy formation. In Section 9.3 we turn to clusters and in
Section 9.4 meet several cases of merging clusters. In Section 9.5 we list possible
candidates of dark matter. As we shall see, there are no good candidates, only some
hypothetical particles which belong to speculative theories.
In Section 9.6 we turn to observations of galaxy distributions and comparisons with
simulations based on the cold dark matter (CDM) paradigm and to predictions and
verifications based on the CDM paradigm.
Introduction to Cosmology, Fourth Edition. Matts Roos
© 2015 John Wiley & Sons, Ltd. Published 2015 by John Wiley & Sons, Ltd.