If the expansion of the universe is continuing unabated, which means intergalactic (perhaps even interstellar) space is continuingly expanding, then:
Why is the Andromeda galaxy on a collision course with the Milky Way?
Surely if the atoms/sub atomic particles of each galaxy were thrown in different directions at the Big Bang then surely they would never collide (especially given the 'existence' of dark energy).
The universe expands continuously but the gravitational pull between Andromeda and Milkyway is still enough to overcome this expansion. That's why there are still celestial formations and even atomic nuclei(the space expands everywhere even inside atoms). Five fundamental forces can overcome the expansion of the universe and hold particles together in an ordered formation. Keep in mind this answer is just an elementary theoretical explanation and there is complex mathematical data that comes with it.
