Supposing an unstoppable object hurtling through space was to hit an immovable object also in space.
Which one would give?
They both vanish in a violent explosion. It’s widely believed that this provides the explanation for the Big Bang.
What’s less well known is that the two objects traditionally engage in a round of rock-paper-scissors just prior to the impact. If the immoveable object wins, the resulting universe will eventually collapse upon itself; otherwise, it expands forever.
Depending on the frame of reference either object could be described as both the ‘immovable’ or the ‘unstoppable’. If you imagine you are riding on the ‘unstoppable’ you actually see the ‘immovable’ hurtling towards you.
From a 3rd frame of reference it could also be seen as merely two ‘unstoppables’ moving at each other.
In my thought experiment the two ‘unstoppable’ objects simply passed through each other without any kind of interaction. YMMV with real matter.
This is an interesting philosophical question, and may be better suited for Great Debates.
In real terms, I don’t think it’s possible to have anything that is either “immovable” or “unstoppable” given enough force.
The unstoppable stops. The immovable moves.
(with apologies to Iain M. Banks, although I’m sure he got that answer somewhere else) 
The unstoppable goes through the immovable.
You can have either an immovable object or an unstoppable force, but to posit that both exist leads you to an immediate contradiction.
An impossible situation. By definition, the two objects cannot exist in the same universe.
Of course, neither are viable in our universe. We could dream up all sorts of scenarios, but it gets kind of boring and tedious when you realize it’s all fantasy and just the usual comic-book fare.
In our reality, you’d actually get something like this.
…If this gets moved to GD, I might offer some fun scenarios though.
Well I did say suppose
Offer away. As a GQ, this seems to be stalled.
The existence of either an unstoppable force or an immovable object posits an infinite amount of mass/energy. I don’t think that sits well with current notions about conservation laws or how the Universe came into being, but I’m not a cosmologist. (I understand infinities are why God invented renormalization, but I’m not sure it applies here.)
Well, then you’re asking a philosophical question, not a GQ question.
Or, if you want to suppose a GQ answer, then the answer is “purple.”
When they collide, they would instantly be converted into a burrito too hot to eat.
In the interests of science can a kindly Mod move this to GD
Exactly. An immovable object, by definition, cannot be accelerated by anything at all. An unstoppable force, by definition, accelerates anything it meets. The two cannot both exist in the same frame of reference.
You might as well suppose that triangles have four sides. There is a useful debate here, but it has nothing to do with physics, it’s a matter of semantics and definitions. I’m not saying that’s a bad thing, we have to know exactly what we’re talking about, but the question you asked cannot be answered meaningfully because of the fundamental contradicting assumptions.
There is no Great Debate here. There is no such thing as an immoveable object, or an unstoppable object.
An unstoppable object would have to have infinite energy, an immoveable object infinite mass. Both are impossible, and so the question is meaningless.
For the non-scientifically inclined, this is like asking “Who are the characters in Shakespeare’s Terminator 2?” Shakespeare never wrote a play called Terminator 2 - it doesn’t exist. The question is meaningless.
However, you might be interested in what happens when two objects with very high energy collide head-on. Their extra energy is turned into mass, and a shower of various subatomic particles is produced. This is what happens in particle accelerators - the bigger the particle accelerator, the more energy it can impart to the colliding particles - and thus, the more massive (and interesting) the particles produced as a result of the collision.
Okey dokey.
Moved at request of OP.
Colibri
General Questions Moderator
But you can’t suppose this. An immoveable object, by definition, is an object that cannot be moved by any object in the universe. An unstoppable object, by definition, is an object that will move any object in the universe. You can’t have both of these in the same universe. It’s like asking if the biggest object in the universe was put next to the other biggest object in the universe, which one would be bigger.
It seems to me that if an Unstoppable hit an Immoveable, the particles that compose the Unstoppable would be forced between the unmoving particles that compose the Immovable, and be on their way as Unstoppable monatomic dust. The Unstoppable isn’t indestructible after all; just unstoppable; whereas the Immovable IS indestructible, because destruction would require movement.