Extreme-energy cosmic ray
EECR's are these cosmic rays that shouldn't really exist -- their energy should dissipate. So the fact that they exist, near us, (though are rare), hints that we don't know something.
Whether it hints at a near-earth Super-Nova that we have no evidence of, or there's some part of physics that's still misunderstood, isn't yet known. But there's something here to ponder.