The concept of dark matter in the universe is by now well established (see  for a collection of recent reviews) but poses still a remarkable problem thereby inspiring the creativity of astronomers and physicists. Nowadays there exist two separate dark matter problems, the baryonic and the non-baryonic dark matter problem (including a non-zero vacuum energy). That separation is nicely visualised in  from which Fig. 1 has been taken. The separation is founded on the well established constraints on the allowed amount of baryonic matter in the universe from primordial nucleosynthesis (see  and references therein). Every hint from measurements, typically on large astronomical scales, for a mass abundance above the allowed one is actually an evidence for non-baryonic dark matter.
Figure 1. Shown is a summary of astronomical results for the mean matter density in the universe combined with a conservative estimate from primordial nucleosynthesis as a function of the Hubble constant. The dark dividing line titled B in the middle gives the allowed amount of baryonic matter in the universe (the lower band gives the amount of visible matter). The gap between B and the observed matter density on large scales (summarised as everything above 0 = 0.3) represents the non-baryonic dark matter motivation (from ).
As more refined measurements from experimental cosmology are collected, like from distant supernova Ia searches, large scale flows or cosmic microwave background , the question of existence of dark matter shifts rather to the question of abundance and nature of dark matter, especially for its non-baryonic part. That part in particular is one point where particle physics and astrophysics merged to form the field of astroparticle physics. Plenty of candidates for non-baryonic dark matter have been proposed. Here it should be noted that the accumulating evidences for the existence of a finite vacuum energy, like a cosmological constant, do not render a non-baryonic matter contribution unnecessary. In fact, the numerous candidates which are classified as cold dark matter (non-relativistic thermal or non-thermal relics from the early universe) are still necessary ingredients for structure formation in the universe .
The favourite particle candidates for non-baryonic dark matter in terms of experiments aiming at their detection are the axion and the neutralino. Since there exist extensive reviews about particle candidates and in particular about the axion and the neutralino, an introduction of these main candidates is considered to be beyond the scope of this review (see e.g. ). They can also be classified as WIMPs (weakly interacting massive particles) together with less prominent candidates (axinos, gravitinos, etc.). In fact, from the experimental point of view the term WIMP summarises almost all necessary requirements for a dark matter particle candidate. Any neutral, massive (between a few GeV and a few hundred TeV for thermal relics) and weakly interacting particle can represent a good candidate. Experiments aiming at the detection of particles with properties as above are described below (1) (compare also the recent review ).
excludes the specialised experiments aiming at the detection of axions.
For a collection of recent references on this topic, see
1 That excludes the specialised experiments aiming at the detection of axions. For a collection of recent references on this topic, see  Back.