Space (between galactic groups) expands. Right now (since the 1990's) there is observed to be an expanding, accelerating universe. Forget about what you might have learned before the late 1990's. Observations have shown that the universe is now observed to be accelerating in its expansion (due to standard candles of Type 1a Supernovae). The expansion is mainly occurring inbetween large galactic groups. Sooner or later, inhabitants of the Milky Way will be unable to see the rest of the Universe, other than members of our Local Group of Galaxies. However, not to worry. We will not be around to see that and Andromeda will have merged with or otherwise disrupted the Milky Way (also disrupting Andromeda), the Sun will have by that time evolved to become a White Dwarf Star and everything will be different.
Sweet dreams...
ljbrs
