Actually it was the Borg. Having gone back in time to conquer Earth, while we are weak and easy, the Borg ship had a magnetic constrictor failure that safeguards the gonculator and prevents the discombobulators from allowing the matter/anti-matter mix chamber to fail, thus resulting in a small anti-matter explosion that destroyed all evidence of the Borg and their ship.

You didn't hear the real explanation from me because only the President and myself know that I am here!!! Shshshshsh