I wonder if interstellar and intergalactic space generate new matter as the universe expands. They are the flattest vacuums in nature. If so the universe could expand forever and always have about the same density.
If two particles are born together and space expands between them, I think they would be less likely to find each other again and annihilate. It would create a bias toward creation over destruction. Multiplied by the volume of interstellar space, it would be very common.
I guess a material question is, if the universe is expanding and more space is being made, is vacuum energy diluted, or does its density stay the same?