  The nature of dark matter (DM) and dark energy (DE) which is supposed to
constitute about 95% of the energy density of the universe is still a mystery.
There is no shortage of ideas regarding the nature of both. While some
candidates for DM are clearly ruled out, there is still a plethora of viable
particles that fit the bill. In the context of DE, while current observations
favour a cosmological constant picture, there are other competing models that
are equally likely. This paper reviews the different possible candidates for DM
including exotic candidates and their possible detection. This review also
covers the different models for DE and the possibility of unified models for DM
and DE. Keeping in mind the negative results in some of the ongoing DM
detection experiments, here we also review the possible alternatives to both DM
and DE (such as MOND and modifications of general relativity) and possible
means of observationally distinguishing between the alternatives.
