Most of the mass in the universe is in the form of an unknown type of dark matter. The need for dark matter has become more and more clear since the 1930s, with evidence from rotation curves, gravitational lensing, hot gas in clusters, the Bullet Cluster, structure formation, and the cosmic microwave background. A consensus picture has emerged, in which the dark matter contributes 26% of the overall energy density of the universe. Its nature is still unknown. At most 15% of the dark matter in galaxies can be white dwarfs (or other MACHO candidates), but most is likely to be an exotic particle candidate. Dark matter searches for the best motivated candidates, axions and WMPs are ongoing and promising over the next decade.
The interesting unexplained signals that may herald the discovery of dark matter have been reviewed: DAMA's annual modulation signal and the Fermi-LAT gamma-rays from the Galactic Center might be due to WIMPs, a 3.5 keV X-ray line from various astrophysical sources is possibly from sterile neutrinos, and the 511 keV line in INTEGRAL might be due to MeV dark matter. All of these would require further confirmation in other experiments or data sets to be proven correct. In addition, a new line of research on dark stars was reviewed which suggests that the first stars to exist in the universe were powered by dark matter heating rather than by fusion: the observational possibilities of discovering dark matter by finding these stars with JWST data were discussed. The goal of the searches over the next decade is to decipher the nature of the unknown dark matter.
Acknowledgments
KF would like to thank Luca Visinelli for commenting on the draft. KF acknowledges support through a grant from the Swedish Research Council (Contract No. 638-2013-8993). KF acknowledges support from DoE grant DE-SC007859 at the University of Michigan.