

It was argued in section
2.2
that there is a clear need for dark matter. In addition, it
seems the baryonic density must be very close to the maximum
allowed by BBN. This last fact raises another interesting
requirement in that the current best estimate places the baryonic
density in the visible content of galaxies and cluster gas
as [103]
. This is well below the range given in table
2.2
and implies that there is a lot more baryonic matter yet to be
found. This missing baryonic matter is also generally referred to
as ``dark matter''. In the following sections the possible forms
of both the baryonic and non-baryonic dark matter will be
reviewed. Table
2, adapted from Carr (1990) [30
], summarizes some of the types of objects that have been
suggested as dark matter.
Table 2:
Possible dark matter candidates (adapted from [30
]).
Figure
3
illustrates the scale-lengths on which the various dark matter
candidates might be significant. The white areas are the allowed
ones. The dark grey areas are strongly disallowed on theoretical
grounds, whilst the lighter grey areas are unlikely but not
rigorously excluded. In the next two sections a very brief
discussion of the main baryonic and non-baryonic candidates is
given. Carr (2000) [31
] and Turner (1999) [138] have given more detailed reviews.
Figure 3:
Possible scale-lengths where different types of dark matter
might be present, based on a similar representation which
appeared in [30].


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Experimental Searches for Dark Matter
Timothy J. Sumner
http://www.livingreviews.org/lrr-2002-4
© Max-Planck-Gesellschaft. ISSN 1433-8351
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livrev@aei-potsdam.mpg.de
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