This phenomenon is just what it sounds like: When two galaxies run into each other, one eats the other. Planets go crashing into each other and get sucked into supermassive black holes, and solar systems get "ejected" from each galaxy. The term "clusterfuck" was invented to literally describe what happens in these cases.
This, on the other hand, is Galactus Cannibalism.
Now, the closest known galaxy that will probably run into ours is the Andromeda Galaxy, which is sauntering toward the Milky Way at a lazy 120 km/sec. In case you just ran into the other room screaming, "SHIT! PACK OUR SHIT!" get back in here and relax. At that speed, they won't run into us for about 3 billion years.
However, there are galaxies out there we can't see. They are called dark galaxies (or "ninja galaxies" for the purpose of this article) and they are made up solely of dark matter and black holes. One of these silent killers could run into us any ol' time now.
What You'll See
We've never observed this happening before, but scientists have theorized that it'll be about the most spectacular and flashy goddamned thing ever.
Like this, but you'll be dead.
Planets will go hurdling through space, supermassive black holes will eat solar systems and shit out Quasars. It'll be a billion Michael Bay movies playing out across the heavens at once.
There is a mysterious force operating in the universe called "dark energy." If that sounds scary, it's because astrophysicists like to come up with scary names for things that are actually code for "We have no idea what this is."
So, aside from being the next name for your death metal band (as soon as you find a drummer), dark energy currently spends its time hurdling everything in the universe away from itself, like everyone did that time you pooped in the pool.
More importantly, scientists say dark energy may be responsible for the Big Bang and creating the universe. And they say it might not be done. Dark energy may, at any moment, decide play out the Ghostbusters "cross the streams" scenario and randomly split every atom in the universe.
What You'll See
An astrophysicist calmly stated, "It's like turning off all the lights... it's the end of time, the end of the universe."
While you're trying to wrap your head around that, just remember that if it happens, you won't care. You won't care because you won't exist. It could happen before you finish your next sentence.
Why aren't they doing something about this, dammit? It's time to send out a ship, find the Dark Energy, and blow the shit up.
In case life-ending threats from the far reaches of space didn't make you shit yourself, have a look at the ones here on Earth that'll do the trick just as well in 5 Bizarre Ways the Weather Can Kill You Without Warning. When you're through with that, learn how to deal with your grief in 7 Methods for Coping with Tragedy (Courtesy of James Bond).