I was reading Phil Plait's 'Death from the skies' where it describes the fate of the universe. It describes the period from 10^90 years to infinity as the "dark age" when all the of the black holes have disappeared.
In this state the universe consists many particles. If two like-charged particles encounter each other, they are repelled. However, if two unlike-charged particles come close to each other, they will move towards each other and disappear to become light energy.
This wasn't covered in the book, but unless I've misunderstood things (which is quite likely), there will be a point when all of the mass in the universe has been reduced to zero after all the particles have paired up and become photons?
If so, is there an estimate when this event will occurr?