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Why isn’t dark energy considered the “fifth force” in the way gravity, electromagnetism, weak nuclear, and strong nuclear are forces?

Tim Smith, Kent, Washington
dark-energy
Chandra X-ray image of dark energy. NASA/CXC/SAO/A.Vikhlinin et al.
Cosmologists are not sure if dark energy is a new force, a modification of known ones, or even perhaps some kind of matter with bizarre properties that fills all space.

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