OK, let me rephrase what you said to see if I understand it correctly. Both particles must annihilate something. The one that fall into the black hole annihilates a piece of the black hole equal to its mass, thus decreasing the mass of the black hole, and the other particle also annihilates an amount of mass equal to its own somewhere outside, so the overall mass of the universe remains the same.

Wait a minute, wouldn't that mean the overall mass of the universe is reduced by the mass of the 2 particles? Hmm, it seems the universe's mass + energy is slightly increased, though. Then again, if the particles had annihilated normally, that would have increased the total energy of the universe, too. I think it was mentioned in another episode that that energy might be an explanation of dark energy.

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"For the world is hollow and I have touched the sky"