A Hubble image of galaxy cluster MACS J0717, which contains a huge amount of information about the cluster itself thanks to the light from background galaxies. Image credit: ESA/Hubble, NASA and H. Ebeling.

Galaxy clusters prove dark matter’s existence

You don’t have to detect a particle to know that dark matter is real.

“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.

Traceable stars, neutral gas, and (even farther out) globular clusters all point to the existence of dark matter, which has mass but exists in a large, diffuse halo well beyond the normal matter’s location. Image credit: Wikimedia Commons user Stefania.deluca.

But 40 years prior, Fritz Zwicky observed the motions of individual galaxies within clusters, and found the same effect.

The Coma cluster of galaxies, whose galaxies move far too quickly to be accounted for by gravitation given the mass observed alone. Image credit: KuriousG of Wikimedia Commons, under a c.c.a.-s.a.-4.0 license.

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.

This image illustrates a gravitational lensing effect due to the distortion of space by mass. Image credit: NASA, ESA, and Johan Richard (Caltech, USA); Acknowledgements: Davide de Martin & James Long (ESA/Hubble).

The key to understanding gravitational observations arises from gravitational lensing, where mass bends the background starlight.