Why doesn't the galaxies collide due to gravitational attraction and why doesn't all the matter in the universe stick together due to gravitational attraction?
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Galaxies do collide, but it takes forever. Read about the Big Crunch scenario for the second part. – Sam Dec 31 '19 at 13:01
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The universe is pretty big and is expanding. Kinda hard to have everything fall back to the same point when it's moving apart faster and faster. – Kyle Kanos Dec 31 '19 at 14:07
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1Related though not exactly a duplicate: How can galaxies collide if everything moving outwards. – John Rennie Dec 31 '19 at 19:00
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1Essentially a duplicate of Why doesn't the Moon fall onto the Earth? – Qmechanic Jan 09 '20 at 11:11
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1@Sam The Big Crunch is extremely unlikely to occur in our universe, due to the accelerating expansion of space, which slowly increases the proper distance between the galaxy clusters. But in the distant future, all the galaxies in our Local Group will most likely merge into one big elliptical galaxy. – PM 2Ring Jan 09 '20 at 11:41
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@PM 2Ring That is why I used the word "scenario". I mentioned it because it addresses the OP's question. – Sam Jan 09 '20 at 13:23
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Galaxies do collide due to gravity. They don't stick together exactly, because they're not solid bodies; but they do often merge. See this article on the NASA website, for example.
Glorfindel
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S. McGrew
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1And, of course, just because things are attracted to each other doesn’t mean they have to collide. Our solar system shows that quite well. – Jon Custer Dec 31 '19 at 16:49
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also, galaxies are mostly empty space. when they do collide, they do not "smash into" each other- they mostly pass right through each other, and disrupt each other's motions via gravity effects. – niels nielsen Dec 31 '19 at 17:27