Excellent question. I want to know too.

The answer is gravity

I don’t know for sure, but my intuition on this would be picturing the universe as a billiards table with balls bouncing around. Even if you were able to increase the size of the table, the balls still might collide together. In other words, expanding the “table” and things colliding aren’t mutually exclusive, though as the table grows beyond a certain point, it will become less common for a collision.

Apples still fall from the trees due to local gravity. It’s only at vast scales when you look at the universe as a whole that the expansion of the universe is the predominant effect.

Because Andromeda is close enough for gravitational effects to pull it together with the milky way. The milky way has collided with others in the past. Frankly, we are lucky to live in a time where other galaxies are still visible. If humanity came later we would think our galaxy is the entire universe as other galaxies would not be observable.

”I’ve heard that most galaxies are moving away from each other, so why is Andromeda moving closer to us?”
– Because those two galaxies are moving toward each other?

Why would you think this would be a paradox? There are OVER 100,000,000,000 galaxies. If “most” of them are doing something, that implies quite a few are NOT doing that thing.

Eventually even protons will exist in in isolation cut off from the rest of the universe and even they may be pulled apart by cosmic expansion.

If the universe is in expansion that means things are moving away from some given point. But if Andromeda and the Milky Way are in tangent paths, they could eventually hit the same point. Think of a triangle. If the two sides of a triangle going from the base to the point, they are moving away from the base but are moving towards each other.

Expansion means things are moving away from all given points as on the surface if an inflating balloon and not just from a particular point.

I enjoyed reading these responses because I was mistakenly thinking that everything was expanding from a central point, in which case it wouldn’t make sense that the galaxies could collide. But if it’s not necessarily one central point, then it makes a lot more sense.

Local gravitation attraction is a stronger force than dark energy.