In my Cosmology course it is stated that the space between galaxies increases with time whilst the size of galaxies (or gravitationally bound space) does not increase. To me, this seems quite unnatural. I understand that the space between galaxies is much larger than the radius of a galaxy and that the increase in the radius of a galaxy may be imperceptibly small, however it seems odd that the space inside a galaxy obeys different physical laws to space outside. And how do we define the edge of a gravitationally bound system, isn't the gravitational force infinite in range? What is going on here?