The Joker thought he had found the answer while dangling at the end of The Dark Knight, but have you ever really wondered what would happen when an unstoppable force meets an immovable object? The folks at Minute Physics did, and as usual brought a mountain of entertaining science to explain how such an interaction would unfold.
The short and spoiler filled answer is: not much at all. The laws of physics would require both to have an infinite amount of mass and energy, but would not allow their velocities to change, so in theory they would simply just pass right through each other. Yawn. No catastrophic universe-ending explosions or anything like that, which is probably for the best. [YouTube]