I have a problem understanding where does the matter/radiation go to because of the expansion of our universe. Eventually all normal matter and radiation will leave our causal patch, but the word leave is pretty unconvincing in this case. They will stay in their own causal horizon until each particle gets turned to radiation and the radiation also leaves the horizon, but how is this synonimous with emptying out. Speaking globally, if the matter density as a whole decreases, where does the matter go? How will the universe as a whole/globally be empty?