At the end of the day, no matter how confident we are in our observations, our experiments, our data, or our theories, we must go home knowing that 85 percent of all the gravity in the cosmos comes from an unknown, mysterious source that remains completely undetected by all means we have ever devised to observe the universe. As far as we can tell, it’s not made of ordinary stuff such as electrons, protons, and neutrons, or any form of matter or energy that interacts with them. We call this ghostly, offending substance ‘dark matter,’ and it remains among the greatest of all quandaries.
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