If dark matter is fundamentally different from the normal matter we know, there should be a way to test it. Here are the results.
Dark matter — despite the enormous indirect evidence for it — sounds like a colossal misunderstanding.
It’s clear that data from
- gravitational lensing,
- galaxy clustering,
- individual galaxies,
- and the cosmic microwave background,
all require masses that don’t interact electromagnetically.
However, a longstanding alternative suggests modifying gravity could explain them without dark matter.
In 2005, a team of astronomers devised a clever test to investigate dark matter’s existence.
When two galaxy clusters collide — a cosmically rare but important event — its internal components behave differently.
The intergalactic gas must collide, slow, and heat up, creating shocks and emitting X-rays.
If there were no dark matter, this gas, comprising the majority of normal matter, should be the primary source of gravitational lensing.
Instead, gravitational lensing maps indicate that most of the mass is displaced from the normal matter.
This remains true for every set of post-collisional X-ray clusters ever measured.
Only if gravity is non-local, or gravitating where the matter isn’t, could the Universe not contain dark matter.
But in pre-merger clusters, we clearly see that gravity is local: matter and gravity line up.
Colliding clusters cannot obey different gravitational rules from non-colliding ones.
Inescapably, dark matter must therefore exist.
Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.