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  . This is well below the range given in Table 1 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) , summarizes some of the types of objects that have been suggested as dark matter.
|Planck relics||Neutron stars|
|Primordial black holes||Stellar black holes|
|Quark nuggets||Very Massive Objects|
|Shadow matter||?||Super Massive Objects|
|Cosmic strings||?||Cold diffuse gas|
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)  and Turner (1999)  have given more detailed reviews.
© Max Planck Society and the author(s)