Variable-Mass Dark Matter and the Age of the Universe
  Models with variable-mass particles, called VAMPs, have been proposed as a solution to the cosmic coincidence problem that plagues dark energy models. In this contribution we make a short description of this class of models and explore some of its observational consequences. In particular, we show that fine tuning is still required in this scenario and that the age of the Universe is considerably larger than in the standard Lambda-CDM model. 
Urbano Fran\c{c}a and Rogerio Rosenfeld