Astroparticle Physics

26513 Dark Matter“There is a theory, which states that if everanyone discovers exactly what the universeis for and why it is here, it will instantly disappearand be replaced by something evenmore bizarre and inexplicable. There is anothertheory, which states that this has alreadyhappened.”Douglas AdamsRecent observations have shown that the universe is flat, i.e.,Ω = 1. The baryonic matter constitutes only a small fractionon the order of 4%. The main ingredients of the universeare presented by material and/or energy that one can onlyspeculate about. 23% of the matter in the universe is darkand 73% is given by dark energy (see Fig. 13.1). There isno clear idea what kind of material dominates over the wellknownbaryonic matter. Even the visible matter constitutesonly a small fraction of the total baryonic matter.The following sections discuss some candidates for theunseen baryonic matter and offer some proposals for themissing non-baryonic matter and dark energy.energy contentof the universe13.1 Large-Scale Structure of the Universe“There are grounds for cautious optimismthat we may now be near the endof the search for the ultimate laws of nature.”Stephen W. HawkingOriginally it has been assumed that the universe is homogeneousand isotropic. However, all evidence speaks to thecontrary. Nearly on all scales one observes inhomogeneities:stars form galaxies, galaxies form galactic clusters, andthere are superclusters, filaments of galaxies, large voids,and great walls, just to name a few of them. The largescalestructure has been investigated up to distances of about100 Mpc. On these large scales a surprising lumpiness wasfound. One has, however, to keep in mind that the spatialdistribution of galaxies must not necessarily coincide withthe distribution of matter in the universe.Fig. 13.1Illustration of the relative fractionsof dark matter, dark energy, andbaryonic matterinhomogeneitiesof the universe