The Dark Side of the Universe

Sammanfattning: The dark side of the Universe presents astroparticle physics with its most outstanding challenge to date. As observational cosmology in recent years has reached a level of unprecedented accuracy, we have learned that there is very little in the Universe that we know much about, and very much about which we know very little. Even though dark matter and dark energy combined is believed to account for more than 90 percent of all energy in the observable Universe, we have no solid understanding of the nature or origin of these constituents.In this thesis we focus on the possible connection between dark matter and extra spatial dimensions. A five-dimensional model of universal extra dimensions is reviewed, with emphasis on the particle dark matter candidate it contains in the form of the lightest Kaluza-Klein particle (LKP). We derive the expected spectrum of high energy gamma rays in such a model and find that it matches recent observations of the galactic center very well, for energies below the LKP mass. However, the mass required to fit the full range of data seems to be ruled out by the relic density bound. We then discuss the cosmological evolution of homogeneous extra dimensions and highlight the difficulties of stabilization in a matter dominated Universe.Also included is a short study of the prospects of using high redshift supernovae to probe the nature of dark energy in the context of quintessence.