"That's nice and all, but I want some proof."

You are a true skeptic, and I admire that. To start with, there is a degree at which light is bent by clusters and galaxies that suggests that approximately 90% of matter is missing. In the General Relativity Einstein predicted that light can be bent at the presence of a gravitational field. This is called gravitational lensing. Any galaxy/galaxy cluster can act as a gravitational lens. Also, the density of the Universe is just about enough to make the Universe flat.

"That is convincing."

PRETTY

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