Wasn't aware there was any controversy about this. I always thought it was believed to be a meteor or comet.
Of course, I underestimated the power of human imagination. I shouldn't be surprised that some people out there thought it was OMG ALIENS or maybe a strange dark matter bomb placed by the Romulans.
After all, if there's a needlessly complicated, idiotic rationale for how the Pyramids have straight walls, there must be one for a giant explosion in Siberia.