I've seen it described and taught many times that from the big bang, space is expanding and all parts expand equally. Almost like a balloon if you put marks on it, they will move outward equally and never touch each other.
But then I hear about colliding galaxies such as, "the Andromeda galaxy is racing toward the Milky Way at a speed of 250,000 mph â fast enough to circle the world in just six minutes. And it's scheduled to collide, head-on, with the Milky Way in approximately 3.75 billion years."
How does expansion fit it to that? Based on the rubber band, balloon, etc explanations for expansion, it should be impossible that two galaxies collide no?