The density of the entire universe is decreasing as an average, but of course the density of the Galaxy, Solar System, or my PC are not decreasing. You could theorise some kind of Absolute Cosmic Clock for the Trekverse, held within atomic structure presumably, giving a measure for Absolute Time (implying Einstein was wrong, time is not just another dimension, etc) but I'm not sure why the universe's average density would affect this, since within an atom gravity isn't an important force - especially the gravity of faraway galaxies! 
