Einstein doubted that black holes, as predicted by the Schwarzschild solution, are real. Some now conjecture that black holes do not exist as such but are dark energy, or that our universe is both—a black hole and dark energy. The Schwarzschild solution of Einstein's equations can be maximally extended to predict a black hole having a flip side—another universe emerging from a white hole. Perhaps our universe's big bang was half of a "big bounce", something's collapse down to a black hole, and our universe popping out its other side as a white hole.