The Universe is dark, but the distorted light reveals its presence.
When we look at the objects in the Universe, the mass just doesn’t add up.
All the normal matter in the Universe — atoms, plasma, stars, black holes, etc. — can’t explain what we see.
There needs to be more mass than normal matter alone to explain what we see.
Some novel, exotic, invisible “dark matter” is the leading theoretical idea.
It’s a radical proposition to presume dark matter’s existence, but gravitational lenses can reveal it.
In Einstein’s General Relativity, the presence of mass curves the fabric of spacetime.
A large collection of foreground mass distorts the background light through the process of gravitational lensing.
When light magnifies and creates multiple images of distant objects, that’s strong gravitational lensing.
These strong lenses indicate six times the total mass as normal matter alone.
They also reveal dark matter substructures: small sub-halos embedded within larger structures.
Additionally, weak gravitational lenses reveal dark matter as well.
These less ideal configurations still distort the shapes and orientations of background galaxies.
By observing this light, we reconstruct the foreground masses of these weak lenses.
In both cases, dark matter is absolutely necessary.
The same ratio — 5 times the abundance of normal matter — explains everything we’ve observed.
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.