The Millennial

When a sufficiently advanced WMD goes off in an unpopulated area of the galaxy, and there's no one left to call foul, does it still make a sound? If no one sees you save the world, does it still count? Earth was collateral damage in a war we knew nothing about. One day we were here. The next a new form of energy flooded our world. Something never seen before, with aftereffects we never could have guessed. It made our wildest dreams come true. Phenomenal cosmic power, itty bitty understanding.  We opened a gate to explore somewhere that could host life, why spend eons traveling the stars when a new dimension was a gate away? This was widely considered to be a bad idea. Thankfully a true hero sacrificed himself by going through the gate to close it from the other side, forced to spend the next few moments experiencing a truly horrible death.  Sadly no one had any idea and the world moved on.

A little while later, he comes back. This is that story.