“You may hate gravity, but gravity doesn’t care.” –Clayton Christensen
In the 1970s, Vera Rubin’s observations showed galactic rotation was too quick at the outskirts for normal matter alone to explain.
But 40 years prior, Fritz Zwicky observed the motions of individual galaxies within clusters, and found the same effect.
Even as we’ve learned to observe gas, dust, plasma, failed stars and planets, normal matter only explains 15% of the gravitational signal we see.
The key to understanding gravitational observations arises from gravitational lensing, where mass bends the background starlight.
Under serendipitous configurations, background galaxies are deformed into arcs and multiple, distorted images.
This phenomenon — strong lensing — allows us to determine the total cluster mass.
Even without optimal configurations, weak gravitational lensing causes a well-defined distortion in the shape of background galaxies.
With numerous enough galaxy counts — obtainable anywhere with deep telescope observations — the total mass of any galaxy cluster can be reconstructed.
Consistently, about five times too much mass is needed compared to the existing normal matter.
Moreover, when galaxy clusters collide, mass reconstruction from weak lensing shows an inescapable separation from normal matter.
No alternative gravity theory explains all this. We need dark matter.