If the universe is expanding and the distances of each galaxies get larger over time then why is our own galaxy (Milkyway) will collide with another galaxy (Andromeda) in the future?
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It really depends on how far the galaxies are from each other in comparison to the force of gravity that the galaxies experience.
Our current theories are that [Dark Energy](https://en.wikipedia.org/wiki/Dark_energy) is a force that exists everywhere uniformly in empty space and its force is felt greater the further apart you are from other objects which is the anti-thesis of gravity where objects feel gravity more strongly the closer they are to a massive object.
To help you visualise the force of gravity, think about black holes and spaghettification. The closer you get to a black hole the stronger the effect of gravity on your body and black holes take it to the extreme where different parts of your body experience gravity differently with the parts of your body that are closer to the black hole feeling gravity much more than the parts further away which will serve to stretch you out like spaghetti.
The force of Dark Energy can be likened to an infinite length invisible **inverse spring** where the more compressed the spring is the **less** force it exerts but as the distance between the two objects it is pushing increases, the strength of the spring increases and while it stretches out it stretches space along with it.
Putting these two ideas into one, we can see there is a battle between gravity and dark energy. If the force of dark energy < gravity force, then two objects will inevitably come together as in the case between the Milky Way and Andromeda. However if the objects are far enough away then dark energy > gravity force and the distance between the objects will increase but not because they are moving away from each other but because the *space between them* is being stretched by dark energy.
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