Assessing the options

The fate of the universe is governed by the balance between two forces—the outward push of expansion, and the inward pull of gravity. Today we know that the universe is still expanding with energy it gained in the Big Bang, and the rate of this expansion has been measured. The strength of gravity, meanwhile, depends on the amount of matter in the universe, and astronomers know that a substantial amount of this is undetected "dark matter." The best models predict that the amount of dark matter is enough to just balance expansion. If these were the only factors involved, gravity would slow expansion almost to nothing, but not reverse it.

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