I'm trying to think of what kind of cosmic event would cause an explosion powerful enough to "shake" or disrupt the whole flow of a galaxy and on that same note what other kind of super cosmic event would have the power necessary to be felt across the known universe?
Gravitational waves are the closest you will get to universe-shaking.
Predicted by Einstein and proven in 2015, these waves are caused by colliding black holes that stretch the fabric of space-time. Unfortunately, that is the closest you will get to shaking the universe. These waves require extremely sensitive lasers stretching miles apart to detect. All told, these waves change the size of the universe by only centimeters. Also, the shaking effect is not instant. The waves move at the speed of light, so they take time to spread through the universe. Even giant supernovae exploding is like a light tickle for a galaxy, and not even noticeable for the universe.
If you are open to sci fi responses, there are two novels from author Schmidt Stanley: “The sins of the fathers” and "Lifeboat Earth". Both tell the story of a very advanced civilization that triggers supernovas regurlarly near the center of the Milky Way in order to create raw materials. But when an accident happens, they create a chain reaction that impacts the whole galaxy. (Actually that accident is relevant to the story but it is not the main topic of the novel).
"But another possibility that has gained traction is the Cosmic Death Bubble.
The details of this death by bubble are pretty complicated, but it’s based on the idea that the universe is metastable, which means it’s not in its lowest or most stable energy state. While we’re okay for now, there’s the (remote) possibility that the universe could drop into a lower energy state, which would set off a giant light-speed bubble that destroys everything it touches."
No real world answer
With our current understanding of physics, there is no kind of explosion powerful enough to shake either a galaxy or the universe. The energy released in a supernova can reach upwards 1e44 Joules, but a (very rough) calculation of the binding energy of our galaxy suggests that it is at least 1e53 Joules (I couldn't find any numbers online, so I ballparked something that is almost certainly a crazy low underestimate). That means that you would need an explosion of at least 1 billion supernovas to blow up the galaxy. It's worth mentioning of course that you couldn't actually "blow up" the galaxy because in practice the vast majority of the energy released in such an explosion would simply fly out of the galaxy since most of it is just empty space anyway (although you could presumably drive away all of the gas and dust clouds in the galaxy, and bake the surface of any and all planets). I doubt you would cause any problems for the stars themselves, but if you blew off enough mass the galaxy may more or less fly apart without sufficient gravity holding it all together (although this is probably also impossible since large galaxies have a high fraction of mass in dark matter, which will not be influenced by physical explosions).
But this is worldbuilding, so who cares. Ignoring our current understanding of physics the answer to both questions is quite easy. You can easily shake galaxies or the universe using just one thing: whatever it was that created our universe in the first place. I'm not personally a big fan of M-theory (aka string theory) but it can be easily repurposed to explain our universe as a small subset of a larger universe with all kinds of exotic physics at play. The idea of the "multiverse" is common enough these days to hardly need explanation, and while it is frequently only used in the sense of people traveling back and forth between universes, the interactions of universes in a multi-verse need not be so benign.
In otherwords, you can certainly come up with any processes you want in the multi-verse that can impact our universe in fun, new, and destructive ways. What happens when our "universe" collides with another one in the multiverse (keep in mind that such a collision would not be a physical one in the sense that we mean these words)? Shaking galaxies? Absolutely! Crumpled space time? Why not! Wiggles in the fundamental constants of the universe? Bring it on! (although that would probably kill all life in the universe instantaneously, so be gentle with that one).
You're not going to get a realistic answer out of "real" physics, but there's no reason to let that stop you.
Well, since vibrations can't propagate through a vacuum, the 'shaking' would have to be something that can propagate through spacetime. Something like a Gravitational Wave fits the bill.
However, any event that would create a gravitational wave strong enough to shake a galaxy would probably also create a flash of radiation powerful enough to boil every planet within thousands of lightyears, and sterilise everything else.
I wish I could give a very elaborate answer here, but the question is just two simple. Respectively:
A galactic collision, that is, two galaxies passing through each other. This kind of event lapses for millions to billions of years. The galaxies may or may not merge. Since space is so empty it is unlikely for stars to actually collide. They are mostly flung to new, interesting orbits, or out of the galaxies altogether.
Big bang and, depending on the models you ascribe to, big crunch (big bang in reverse), collision with another universe, or brane collision. None of these support holywoodian apocalypses, though, because science ruins the rule of cool.
The answer to your question is dictated by the sheer vastness of space, and galaxies, themselves.
To "shake" something,means to cause it to reasonably quickly vibrate back and forth. Making a few quintillion quintillion quintillion tons of matter over many billions of light years, or even on a galactic scale, is going to need immense energy.
To "explode" matter, means to convert it to fragments, or back to energy, or similar. That's also going to take immense energy.
As far as we know, there aren't that many physical causes which might be capable of the scale and type of energy output, suitable for the question. In fact I can only think of three known or seriously believed likely to exist:
- A false vacuum collapse event. This would modify all quantum fields, spreading throughout the universe at the speed of light. It would certainly be capable of "shaking" in one sense, because of loss of stability of all objects. The trouble is, the effect itself propagates at the speed of light, so an object is either unaffected yet but unaware, so it isn't shaken, or it's become "new" quantum fields (if able) and probably not "shaken" either. But this does probably fit the OP's intent, even if technically not shaking.
- A change to the geometry of spacetime. On a "small" scale that might be an insanely vast black hole event or other event capable of creating gravitational waves. On a "larger" scale, there might be ways the entire metric could suddenly change, as seems to have happened during the Big Bang. The problem with the first is, it's very unlikely you'd get an event capable of creating "shaking" on even a galactic scale, which you'd notice outside a laboratory. The problem with the second is, would you get "shaking" at all? After all, when the metric changes, objects will seem to be a different distance, but no physical movement occurs, you won't feel shaking happen anywhere.
- Some vast concentration of energy is released somehow. The problem again is, you need so much, that it doesn't seem feasible to concentrate the energy needed in a way consistent with physics. You might create a black hole by doing so, or blow up a small part of space, but you won't "shake" anything on a scale of even lightyears as far as I can see it, much less a galactic scale. Could you create an antimatter stellar size object, and throw it into a star? That sort of thing would be about the most concentrated form of energy we know could exist and perhaps be assembled in the "real world" (earth size objects made entirely of protons or electrons would be impossible to assemble). Maybe, but while you'd get insane amounts of gamma rays emitted, it's doubtful you'd get "shaking".
Nobody has mentioned quantum entanglement (or rather, I didn't look closely enough, perhaps). The problem with shaking things of cosmic proportions, is the speed of light; entanglement overcomes this, at least in theory. Physicists don't really understand why QM works, which is one reason why we haven't been able to unite it with General Relativity - but that is an advantage from your point of view, because you can construct an advanced science that explains how all particles in a galaxy got entangled so that a certain event can trigger an effect everywhere at the same time.
The Big Rip would count, but then nothing would be left to do the counting. :(
The idea is the opposite of the Big Bang. At some point in the future, the expansion of the universe dramatically speeds up. At first, we would see distant galaxies getting farther away faster. Then nearby galaxies. Then our own galaxy would drift apart. Then, alone in a dimming red gloaming, the planets, our own planet, and even the bonds in our own bodies would be ripped apart and flung wide. Every place in the Universe would experience the same far-to-near timeline.