Add up every star, cloud of gas, planet and human body and you get about 5% of the energy of the universe. Vera Rubin measured in the 1970s that stars at a galaxy's rim orbit as fast as those inside instead of slowing off in Keplerian fashion; gravitational lensing weighs out the same missing mass, and in the Bullet Cluster the mass has separated cleanly from the hot gas. That much is measurement. What carries the mass is still uncaught: WIMPs, axions and MACHOs remain undetected, and the possibility that gravity itself needs fixing has not been ruled out.
Add up every star, cloud of gas, planet and human body and you get about 5% of the energy of the universe. Vera Rubin measured in the 1970s that stars at a galaxy's rim orbit as fast as those inside instead of slowing off in Keplerian fashion; gravitational lensing weighs out the same missing mass, and in the Bullet Cluster the mass has separated cleanly from the hot gas. That much is measurement. What carries the mass is still uncaught: WIMPs, axions and MACHOs remain undetected, and the possibility that gravity itself needs fixing has not been ruled out.
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