Scientists are using all the normal matter in the Universe (atoms, plasma, stars, black holes, etc.) and they still can’t explain what is being seen. This theoretical idea is led by the exotic and invisible concept called, “dark matter.”
They are referring back to an idea of Einstein’s called, “General Relativity,” where the presence of mass can curve the fabric of spacetime. When there is a big amount of foreground mass, it distorts the background light through gravitational lensing.
When there is strong gravitational lensing, the light magnifies and creates multiple images of distant objects. Scientists can also detect matter with weak gravitational lenses.
The lenses used can detect six times the total mass and can also reveal dark matter substructures. When observing the light, we reconstruct the foreground masses of the weak lenses.
Scientists have seen the same ratio in their gravitational lenses, 5 times the abundance of normal matter.
No matter what we see, dark matter is absolutely necessary for the balance of the Universe.