What is the hot subject/topic in particle physics these days?
The answer to this question depends
on what type of particle physics you are interested in.
In theoretical high-energy particle physics, which is the name usually
given to theories attempting to unify the force of gravity with the
other 3 known forces (electromagnetic, strong and weak forces), the
hottest theories are string theory:
Particularly relevant to your question is "The Dark Universe" link, the
fifth link on his list. He describes the candidates for dark matter
particles there. The Chandra Q&A pages also touch on this subject, for