From the January 2011 issue

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
By | Published: January 24, 2011 | Last updated on May 18, 2023
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.